Why doesn't she mention her boyfriend?

There is this one girl in my class that I recently started talking to I initiated the conversation and from there we started talking just about every day in class and we walked with each other to our other classes being they were in the same building. I asked her for her number and she was enthusiastic to give it to me so didn't hesitate at all, but I'm not going to right away think she is all into me she just might be a really nice person. We texted each other once, but nothing to exciting now while we were walking to our next class I asked what she was doing over the weekend and she was telling me how boring it was going to be that she had nothing going on so I suggested that we hang out and do something her response was like "yeah that sounds good, but ummm well I'll let you know" after that I knew she wasn't telling me something, but I wasn't going to push it. Later on that day we meet up with each other and started talking I didn't bring up our earlier conversation so it wasn't so awkward when she received a phone call turns out it was her boyfriend they sort of argued about something I'm thinking he seen her with me and called her to see where she was at and who she was with because she was telling him she was walking to her car and that she was by herself even though I was right next to her. So after she ends the call I ask that your parents to which she replies "noooo my boyfriend" as if she wasn't enthused about it. Now know she had a boyfriend I kept my distance cause that's now how I roll, but I still was friendly to her. I did notice she started texting me more and always initiated the texts and she even started sitting next to me in class. It just bugs me as to why she didn't tell me she had a boyfriend even though I asked her out and trust me folks I would not have been hurt if she just plane out told me she had a boyfriend heck I would of shaken her hand and thanked her for saving me the time and hassle.
Why doesn't she mention her boyfriend?
Post Opinion