Why would my female friend still feel the need to distance herself from me even if she does trust me

I had this very good female friend. She was one of my most supportive, encouraging friends that I have ever had. We used to work at a fast food restaurant together and became good friends there. She is a shy person and I am a shy person as well, so we both related to each other very well. I left the job and me and her kept in touch on social media, texting and so on. All of this time she has had a super, controlling jealous boyfriend and she is also a jealous girlfriend with him. However, me and her were good platonic friends and she trusted me as me and her would always have deep conversations and talk about life. She would ask me for advice whenever she was having problems with her boyfriend and I was always so supportive of her relationship with her boyfriend and I encouraged her to stay with him. Me and her joined a technical college course together as friends and when we told each other we were both going to do the program we were both very excited about it. So we were in school and things were good. We hung out together all the time in school as we always sat with each other and talked all the time. Again, I do not like her romanticly but I care about her as a friend. She would always talk to me about her boyfriend and their problems and one time she even asked me if she should break up with him and I encouraged her to stay with him because I knew she loved him very much. She trusted me that much to ask me that question. We went everywhere in the school together. Whenever, we would be walking in the halls together hanging out whenever one of us had to do something quickly like use the restroom, but something to eat, go print something at the library, pay something at the registration office we would always wait for each other as good friends. However, she told me that she could not tell her boyfriend that I was in the school with her because if he knew then he would get very angry with her and accuse her of cheating. She told me that he has anger problems and the littlest things will set him off on a rant about her cheating on him. However, she told me that if he was in a school with a nice, caring opposite sex friend like me that she would be angry with him as well but assured me that she trusted me. I knew with her I had to limit our texting because of how her boyfriend is and I never texted her too much and we usually could only text late at night when her boyfriend and her were not together. So we started clinicals together at a hospital during the course and whenever she would get to a grassy area parking lot she would text me when she got there and then I would text her too and she would voluntarily text me when she was getting out of her car so that we could walk to the hospital together. She would want for us to walk together. However, all of a sudden she became very cold and different with me. I noticed that now in school she was not sharing stories with me anymore and telling me things. I noticed that she started not liking that I wait for her outside the restroom even though she never had an issue with it before. I decided to wait a couple of more weeks to see if things would go back to normal but things remained. So one day in school I asked her if I am bothering her or if I have done something wrong? She told me that because I wait for her outside the restroom when we are hanging out together in the halls and walking together that she started to get the feeling that I liked her romanticly. I quickly assured her that I did not and she said she believed me and trusted me. I asked her if their was any other reason she started feeling that I liked her as more than friends and she stated it was just the bathroom thing and I reminded her that she seemed perfectly fine with it before and understood why we do it. However, she would not me explain myself that we'll so I had to wait a couple days until I could again reassure to her that view her as a platonic friend who I care about a lot and she is not the only girl whom I have this kind of friendship with as most of my closest friends are women. She said she trusted me. However, she did say that she wants to keep to herself more. During the next week in school there was a moment where I felt maybe she was not being totally understanding and I felt the need to reassure her again and she told me to stop that and that she was getting annoyed. A few days later I sent her a harmless happy thanksgiving text to her that I send to some of my other close friends but she reacted by blocking me on messenger and my number. I approached her about it that next Monday but she told me that I have annoyed her and she was very rude with me for no good reason. I decided to take it easy in school for a couple of weeks. Once a couple of weeks passed I saw things got a little bit better but then I still noticed little things that showed that things were not quite back to normal. She told me that she believes me and trusts me that I view her as a good friend but that she feels it is disrespectful to her boyfriendfor us to be so close and that she knows she would not like it if he had a friend of the opposite sex that was as good as me. She told me she felt that we should not keep in touch because she feels like she is doing something wrong to her boyfriend. I never did anything inappropriate, flirtatious, to her and she knows that. I asked if we could take a selfie together as platonic friends and she said no. I rushed after her to let her know I only meant it platonicly. Again she got annoyed. She took me off of her snapchat but she told me deleted all the guys. I wrote her a letter at the end of the course letting her know I support her and only want the best for her and her boyfriend. She told me she would read it after clinicals. I understand that she wants to respect her boyfriend but she could handled this so much better and just let me know what was going on? I would have liked her to tell me not to message her anymore instead of her just straight up blocking me. I never did anything wrong, she just did a 180 on me. I feel emotionally hurt and I have been beating myself up mentally for this and I really never had any bad intentions with her.