After making out/hooking up with this guy for a couple days I began to get a cold sore on my upper lip. Since I didn't really take care of it and kind of ignored it, it spread to my lower lip. Is it a coincidence that I got it while being with this guy? Or is it something to do with this guy? I did some research on the subject and I still don't really understand- do I have herpes and he activated something in me that made me get a cold sore or does he have herpes and I got it from him? Basically, who's "fault" is it? Who has the problem? And will this kind of thing happen again if we do stuff again together?