girl troubles

ok now i have been going out with this girl for a year and half and i think that she is cheating on me now!! i have gone to college this year and we just have not gotten along but we manage to stay together!! does anybody have any ideas of how to find out!!! i have ask her but she said she is not!! but tonight i called her and she is acting all weird and i ask her what she is doing and why she didn't call me and she doesn't want to awser me!!! what to do?????