Believe me, you don't have to be a man to not understand things like "I love you, so I screw you over in worst ways" and "I wanna spend more time with you so I'll hide away and tell you I'm busy". I'm seriously ashamed of my sistah's sometimes.
Oh, and how old is the chick in question? If she's 17 or so, it's normal. Some of them grow up.
Obviously the only way to know why she is doing what she is doing and how she feels about you and the relationship is to talk to her. Playing games by not answering her texts is just immature and does not help things.
A simple, "hey I noticed when we are together you seem distant and uninterested in being together, is everything ok?" will start a conversation with out hurt feelings.
Sounds like you should marry her. Seriously. How do you feel, you missed that major point in the first post. If you love her, tell her, and keep her, and put up with it. Everyone gets frustrated with their loved ones, it's part of a relationship, otherwise it'd be boring as fuck.