WHy do I feel this way?

I have been waking up in the middle of the night almost every night with the thought of my girlfriend cheating, not even dreams just the thought of it, and Ill wake up think about it, tell my self to stop it then go back to sleep. I havent told her about this because I dont want her to feel like I dont trust her. She hasnt given me a reason to feel like she could be doing anything. Why is it that I feel like this? and feel this way? Its not like Im trying to ignore the signs haha...there isnt any signs lol. And considering my current mental status lol...Id be extra sensitive to any signs, we have 3 times to 4 times a week, I mean everything is good really, but why do I wake up thinkin this every day, its eating me up I dont understand it. I wouldnt want to express this feeling because I dont want her feeling like I dont trust her or that im insecure, I dont know why my mind jumps to this, my thought is to just ignore it till it goes away lol