Dude, try to do it yourself and if that fails, go to the shop.
One thing about girlfriends (or boyfriends for that matter), when you are sleeping with them, you're sleeping with every person they've ever slept with. Do you really want to sleep with her and whoever she's boinking on the side? Pick up her phone and when she goes nuts ask her what she's trying to hide and if she really thinks you're that stupid.
Then watch her try to stumble in the attempt to find a good lie. If she comes off with a clean, smooth line...she's rehearsed it for just such an occurrence and needs to be shot dead (or at least shown the front door). If she stammers around, she's cheating and just isn't smart enough to have practiced a line. If she reacts with speechless stupification, get rid of her because she's just too weird. If she lets you go through her cell phone history, she's one step ahead of you and is deleting it so you won't find out...get rid of her.
Check your PM box for my phone number. We'll go out for a few drinks and I'll help you get over her.