Should I investigate or just wait for disaster to strike?

I've been going with this really gorgeous girl for quite sometime (1 1/2 years) and I really think that we are starting to connect on a much closer level now more than ever. However, recently her ex boyfriend has started talking to her again via myspace/facebook. She tells me that they are just stuck as friends now and that nothing is going to happen. I want to trust her so damn bad but i can't help thinking that she might be seeing him and/or having extra "whoo-hoo" with him behind my back. I've been taking her out alot and talking to her on the phone to kind of make sure that she kinda stays out of contact with him. But now she mentions that I might be getting a little too "protective"of her and that I should trust what we have. Should I go Sherlock Holmes on this mystery of whether or not she's seeing him? OR Should I not worry and trust her until the day comes that proves beyond all doubt that she is seeing him (if she is..)?