Why is she acting like thiss?

So my girlfriend has been making plans with me...and then cancelling them for the past week or so....she keeps saying she has to p her finals and has to graduate and then after that we will have all the time we want and she keeps telling me she is not ditching me...but the excuses she makes are so obviously a lie and it pisses me off so much. I have no idea what to do about it...she keeps telling me she isn't ditching me and wants to see me...for instance...this week she told me she couldnt see me all week...then she said she wanted to hangout tonight. then last night she tells me she can't go because she has a final Friday and she has to study.....then she calls me later at night and she sounds a total mess..like she has been smoking or drinking....I can't tell what is going on...is she seeing someone else?...ditching me to smoke?....studying??....I have no idea...but when she calls me and she is a mess or whatever the truth usually comes out...but she still insists that she is not ditching me