fact is, if you are having these thoughts, you dont trust her. Sorry, but thats the way it is. A better approach than "i trust her, but my brain makes up situations" would be to approach it as "i dont trust her". now that you've admitted that, you can get to the root of the mistrust. Amigirl has alot of good questions to ask yourself. Even if you come to the honest and complete conclusion that everything is in your head, you need to figure out why, because there is something going on not necessarily something shes doing, want to make that clear, but it might be, and you need to figure that out). The only way this would come out of clear air is if you were paranoid schizophrenic. However, what may be going on might be subtle.
for instance (and im just throwing things out there, might or might hit the nail, so if i dont, keep thinking) you might worry because she doesn't tell you where shes going or what shes doing. she might talk to much to other guys...or not enough! like shes hiding something. Or turn it around. You might be thinking about cheating on her, and projecting that feeling so as to justify your own thoughts. Again, if none of this is right, just keep searching.
Whatever the reason, you then need to figure out what to do about it. Remember, trust is earned and learned. Has she earned it? Have you learned it? Or are you just pretending because that is "whats expected"
sorry if this comes out harsh. im in a bit of a foul mood atm, but i mean everything i say with the best intentions....just dont have the inclination to dress it up with niceties right now