Why do girls hook up with girls?

My girlfriend of the past 3 and a half years went home last month by herself to see her family and through some research, I discovered that she hooked up with someone while she was there. When I initially confronted her about the fact if she cheated she said no. I gave her all the chances to come clean b/c I knew something was up she still stuck to the NO!!!...The way I found out something was up was by going through her cell phone...there was a message from her cousin who she claims she was with, advising her of how to cover up b/c it seems she was out all night and her mother was worried. After hours of trying to get the truth she claims she was with a chick she knew from school and all the did was kiss and watched a movie. She claims she fell asleep which is why she got home late. My issue is that I also seen another message saying " If I had known that I wouldn't have let you leave lls. I thought your was good...I cant wait till round 2" (This message came through a few days after her trip)...This was a forwarded message she sent to a friend and the original message and others that coincide were deleted. She claims she was gossiping and it was a message one friend sent to her and she was gossiping with another friend and forwarded it. My thing is it would have a FWD FWD on the message which it didn't it only had 1 FWD. She claims she always erases the first forward on all her forwarded messages(weird). She comes home late sometimes and I think she could possibly be cheating. I jus bought a house this year and need her to help pay mortgage dont want a roomate and would hate to have to sell. Tried giving it some time but I feel no better, partly b/c she still comes home late sometimes....today 4 and a half hours after work (claims she was at a friends house). Thought this could be the woman of my dreams but not sure anymore. I Love her but refuse to be a FOOL!!!!