Don't worry. I had the exact same thing and I've tested negative out to 12 months (my choice -- nothing whatsoever to do with the window period, so don't start that please). I also had some really sticky, gummed up saliva that would collect between my cheeks and gums. Really weird.
Anyway, my guess was that it caused by the extreme stress and anxiety I was experiencing since it cleared up after a while (took a few months though...)