My boyfriends female colleague

My bf works with this girl (I'll call her "K") and I've had a bad feeling about her since she started working with him. He always told me that she was dull but he still talks about her daily to me. She asked him at work if she were his favourite colleague and told him to add her on FB. He told her he doesn't add colleagues on FB and laughed off the other question. I asked him if she knew he had a girlfriend and he said no. I suggested that maybe he should mention it because it sounded like she was flirting. At this point I’d relaxed. If she didn’t know he had a girlfriend I couldn’t be mad at her for flirting with him. A few days after this he casually dropped into the conversation that he had a girlfriend, because he too started to get a feeling that "K" liked him. My bf works with other girls (one especially pretty one) and I’ve never worried about the other girls and still don’t.

One day she asked to see a picture of me, he showed one to her and she apparently sulked the rest of the day because she thought I was "quite pretty" and she was surprised. Since then, she's "joked" to my bf that she wants to get rid of me, that she has a shrine for him and that the thought of him naked is a nice one. He's told me that she rolls her eyes and doesn't like it when he mentions me.

During all this, I got two friend requests on fb from male colleagues. I accepted as I knew them and they instantly started messaging me. I was next to my bf at the time and he was looking at my phone with me. He said he wasn’t comfortable with it. Immediately after he said that, one of them started flirting and asking me to go out for drinks. I told them that it wasn’t appropriate because they knew I was in a serious relationship (we are practically living together now).

About a week ago we were on the bus from work and he got a text from "K". I honestly didn't mean to look, but he opened it right next to me and I was facing that way. I saw that he initiated the conversation, included wink faces and encouraged the conversation to continue. I didn’t want to mention anything on the bus so when we got back to my house I told him that him initiating texts her made me uncomfortable. Because we both know she likes him and he knows how I feel about it and how he felt when it was the other way around. I felt he disrespected my feelings on the subject and it would encourage "K" to carry on the flirting and texting. It resulted in an argument between us, and I started crying. He calmed down and told me that he doesn’t even want to be her friend or socialise with her and promised he wouldn’t do it outside of work when it wasn’t work related.

Since then he has initiated texts with her twice, with a different excuse each time.

I feel so horrible for confronting him over it. I guess I just need advice. Am I in the wrong? What should I do if it continues?