Is my girlfriend cheating?

We have been together for a year.. anyways lately she just got transferred to the loan department at her bank... been working about 11-12 hours a day. So she went out and bought a moped saying she wanted convenience (drive home after work instead of waiting for the bus) I understand that part. But whats weird is even though we hang out like the whole weekend together, she is just not into sex, saying that her period is coming soon.. shes actually not been "sexually right" for awhile now.. before we'd have sex and shed get into it. But now she is barely into it, and last night even joked I should pay her, I took it in stride since she is a joker but I wonder what might be happening.. we joked about her becoming an escort. she is really poor now.. but it was just a joke-- I think.. what might be going on?

oh yeah when we do have sex-- she just keeps saying Im too big and it hurts.. Im her biggest yet, but eve thoguht she has always said Im big... she didn't used to complain I was too big and driving too deep... totally weird

There simply isn't enough info to make an informed decision whether your gf is cheating or not. However, it is obvious that there is some rift in your relationship at the moment. I suggest that you do the hardest thing there is to do...talk to her. It sounds simple, but if you read most of the posts on here, communication seems to be the hardest thing to do.
I wouldn't accuse her of cheating, simply bring up that sexually your relationship seems to be off track lately. Ask her how she sees the situation and what she thinks you two could do to remedy things.
At best I think you have the makings for some great role-playing in the works. She seems to be receptive to the escort fantasy. Go with it and see where things could go.
Best of luck.
Pheniousf