My girlfriend lies a lot!

I have been in a relationship with this girl in school for about 3 years now. sometimes she appears caring, but at other times i get this 'i don't care' attitude from her - like not picking my calls (when she's really not busy) and not bothering to call me later. I've spoken to her about it severally. Each time she says she loves me and that she tries not to show it too much so that she does not get 'too emotional'. lately, we had another problem and i called her up to discuss it. she asked that i call her back in about an hours time (8pm). when i called her back severally, she didn't pick. i thought of the possible excuses she could give me, so i sent a friend of mine to check her room in the hostel to confirm if she was there and if she was sleeping. My friend said her roommates told her she went downstairs (usually going downstairs by that time means going to chat with a male visitor, cos they are not allowed into the hostel by that time). At about 10:10, she called me back. I asked her why she wasn't picking, and she said she slept off because she couldn't sleep the night before. Really, i am not too concerned about any male visitor going to see her, but, i still don't get why she should see someone when she knows we were meant to discuss how to save our shaking relationship. sometimes, i feel she might be cheating on me, and because of her lies (which she somehow is used to), i find it difficult to trust her. i don't know what to do.