cheating

my cousin brought up my girlfriend in the car earlier. he asked if we were serious. i said, "yeah. well at least i am. i dont know about her".
then he brought up about how he got cheated on by his last girlfriend. which totally put the thought into my mind... "what if tia did/is/will... ?" &&what if she's js using me.
not saying that she did/is/will, and im not saying that she's probably using me. js you know. the thought of what if scares me.
not that i don't trust her, because i do. it's just im scared of losing her.