Yeah, it sounds more like a cyst to me based on your (absolutely disgusting, though I guess I did ask for it) description. I’ve got both oral herpes and am prone to both sebaceous and fluid-filled cysts. Did it leave a rapidly-closing cavity? A herpes outbreak would leave a painful, hot, yellow-brown crust that leaks pus when broken. And you definitely haven’t had any cold/flu symptoms recently? I’m not a doctor, and i think it’s a good idea for you to see one about this, but my personal suspicion is a sebaceous cyst.