maybe you're having trusting issues...ask yourself why are you having those thoughts...and if things have been going good since you two started dating, think what's different now..maybe that could help you..and you gotta know that when you really love someone you wouldn't have any doubt in your relationship..
Talk. Ask your partner how he/she feels about where you are at in the relationship and where it is going. If the two of you can't talk openly with each other, you will always have that question, that nagging feeling in the back of your mind. That isn't healthy. If you are happy with your partner and can see yourself moving forward, together, then you need to talk. Have a nice dinner at home. It is easiest to talk openly in a relaxed environment.