Sti???

Lately I’ve noticed that I’ve been having cramps between periods not to bad tho and I have been spotting (it looks like bleeding but there isn’t a lot.. not even enough to get on a pad) between periods. Spotting mostly comes after sex. I’m going next week to get checked but do you think it’s an sti? If so how would I explain this to my bf... I’m so scared I’ve never had unprotected sex till him and we have been trying to convince but I need getting BFN’s and I was wondering if this could be why..