Whilst it is possible that she has cheated, it's also highly likely that she hasn't. I go through periods of having a really high sex drive, and also not wanting it at all - and it's nothing to do with the person I happen to be seeing at the time. It's just me, and it's normal. Most of my girl friends, whether in a relationship or not, long term or short, go through this, although it's more likely, as a rule, after you've been dating a couple of months or whatever and the initial urge to ravish each against a wall or something or other the moment you see each other has diminished a bit ;)
Talk to her. Let her know that you want to please her in bed, and for it not to just be something she "lets you do". Perhaps ban penetrative sex for a month or something, and start from the beginning again? Do a day where you pamper her? Run her a nice hot bath with candles and bath bombs and stuff and then give her a massage? Watch a film together under a duvet, give her a foot rub after a hot long day. Stuff like that that is still initiating contact but in a purely non-sexual way?
Something my friend did for his girlfriend when he had the same kind of problem was make her a book of vouchers that she could "redeem" whenever she wanted to. Like "exchange for a half hour long massage"; "exchange for a foot rub"; "exchange for me cooking a meal for you" etc etc. That way she can "redeem" them when she feels comfortable, but as you've given them to her it's not like she feels she's demanding/asking for it, if that makes sense?
I hope this is more coherant than I think it is
But I just want to re-itterate that I don't think it's likely that she's cheating on you - have faith in her.
Good luck!
Chelsea