Figured it out!

CAFFEINE speeds up your metabolism and suppresses appetite. I used to not be a regular coffee drinker, but since the beginning of this academic year, I've been having a regular morning cup, 3 or 4 times a week.

I couldn't explain the weight loss, because I never lose weight, and I thought that it might be an ulcer. But now that I'm armed with this new information, I'm going to guess that it's been the caffeine.