What is up with my girlfriend?

So I'm a junior in high school and my girlfriend and I have been dating for almost four weeks. We get along pretty well, but we don't really get to see each other a lot because she's in a play and has rehearsal every day after school and sometimes on the weekends. Two days ago I went to her house and we watched a movie. Everything was going well. We were cuddling and it was all good. Then yesterday (Monday), in the morning, I went to her locker with her, and she told me I should go to homeroom because we get yelled at for hanging out in the hallways after 7:45. I said no and blew off the comment because that's never bothered her before. So I went with her to her homeroom for a few minutes before school started, and she told me to leave right when we got inside. Once again, I blew it off thinking it was just because school started in five minutes. Then, in Chemistry later that day, our last cl, she decided not to sit next to me, instead sitting alone a few seats over. That's when I started getting bothered. Every once and awhile I would make a comment and ask her why she wasn't sitting next to me, like she always does. She didn't respond. Then, after Chemistry, I was going to go with her to her locker again, but instead she told me again to go to homeroom, and that my "lost puppy" thing (that's what she calls me when I spend time with her in the morning because I follow her everywhere. She usually says it's cute.) is getting old. That was a stab to the heart, and I made an annoyed face and left. Then I was upset for the rest of the day. I talked to one of our mutual friends and she said that I should ignore her for a day or so and see how she reacts. So I did. I didn't txt her all last night, or this morning. I didn't go to her locker or homeroom in the morning, and I got no response. Then, at the end of the school day, we usually have a period set aside where we do service work in our school office, like bring messages to teachers and stuff like that. She is usually a very talkative person, but today she was silent. It was really awkward and I tried to look angry and frustrated, but once again I got no response. When the bell rang I got up and left without a word. I went to lift weights after school, and I talked to the same friend I talked to yesterday. She said it was just her "drama" and I should talk to her about it. So I sent her txt's asking her what her deal was, and that I was hurt that she'd been avoiding me lately. She hasn't replied yet, but I wanted to see what people thought about what is up with her. She usually isn't this way, except when her birth control pills are acting up, but I don't think that's the problem. I need help!!! So far I can't think of anything I did between Sunday night and Monday that would have upset her, and if it wasn't me she would have told me about it. It's really not like her to be this way.