Cold sore?? Herpes

So I have a bad cold sore on my lip. Started off as like a cluster of mini blisters then it spread and got all dry and my skin is all cracked and red so Ik for a fact it’s a cold sore. However the only person I’ve kissed in almost a year is my boyfriend of 7 months. I’m not jumping to conclusions but if I have been loyal and have not kissed anyone but him that means he gave herpes to me which means he was probably kissing someone else? I’m not jumping to conclusions but this is the only logical thing I can think of. What do y’all think? My bf has cheated before so that’s why I’m thinking this way.