just thought i'd add my two cents...
i started getting them when i was about 15 (huge ones...like a cluster of them the size of a dime!) and quickly realized that it was from citrus fruits (oranges, tomatoes, lemons, limes, grapefruit, etc.). i stopped eating those and i don't get them anymore....well...actually...i get them sometimes when i eat spaghetti bolognese (not nearly as severe as when i was 15 though). when i do, i use Zilactin (US) or Abreva (US) and they go away pretty quickly. if you'd like a natural remedy, you can try using tea tree oil. it doesn't taste all that great, but it works.