Is it normal to think about a girl so much?

I have a girlfriend who was,until relatively recently, very friendly to me. We talked a lot and were almost best friends.I also must mention that she is quite attractive and friendly.Then,suddenly, she stopped talking to me and she became quite distant to me.Now she never talks to me on IM (in the past we talked daily)and when i see her she seems not to really be interested in talking to me.I don't understand why she is acting like this to me so suddenly and it is becoming quite annoying because i often think of her and when we used to be good friends.I wish i could forget her or she could realize that i really care about her.PLEASE give me some advice!i really need some and i want to tell her this some day?is it a good idea?