Ooooh, buddy, you got any new smoochin partners lately? Or share a drink with someone new? I'm no doc but ya might have gotten one of the most common STDs on the earth, oral herpes.

It's fairly manageable, my boyfriend has it and I don't, (and we've been kissing on and off for like a decade and a half) you just gotta be really careful about when you have sores and make sure not to kiss anyone or share drinks until the scabs FULLY heal.

Edit: I might be wrong, though. I usually see herpes manifest as cold sores (ya might call those ulcers in the UK, i dunno), not a pus-filled zit. But, and sorry to say, ya might wanna ask a doctor about it.