ups and downs

My girlfriend and I have been dating for nearly 4 months now. We've been having more and more fights where I feel like I put in significantly more than she does. She disagrees. Today was especially bad. We were very affectionate with each other earlier in the day. But as time went on, we grew apart in the span of the day. I'm not sure how to describe it. Earlier tonight, she felt sick, and I went and cared for her for 3 hours. Afterwards, I asked if she wanted to stay over and just spend time watching something with me and she declined saying she was tired and wanted to go home. I offered to go keep her company till she wanted to go to bed since it was still really early and since we hadn't really spent much time together today doing something together. She accepted. After we got there, she decided she wanted to do homework instead, and I asked if we could do it somewhere private instead of a public study room, she declined. I was frustrated and told her I felt like we grew apart so fast some days and it bothers me. She told me today was just a bad day. After eating with me, she left to go do homework by herself completely opting to stop hanging with me. She told me she had more important things to do than make me feel better about today and promptly left after offering a hug. I guess what I'm asking is, is this a problem or is this normal? I felt insulted when she told me she didn't have time for me when the homework she was doing was due 3 days from now. Am I being too sensitive? Should I cut back on the amount of time I put into us? How do I deal with this situation in general?