Welcome to the club! I had never heard of it, either. From my early teens on, within a few hours of eating watermelon, I would have severe (really severe) pain, gas and diarrhea. Like a dummy, I didn't specifically blame it on the watermelon. I wondered about it, but assumed that no one could be allergic to this innocent and refreshing fruit.
When my youngest daughter refused to eat watermelon (See Be sure allergy isn't the reason your child won't eat certain foods) I laid it to contrariness. At that point I had cut back on my consumption of watermelon also, although I still wouldn't believe it was an allergy. When an allergist diagnosed melon as one of the items my daughter was seriously allergic to, I was astonished. I went out and bought one, ate a number of pieces, and.... ouch!!! My daughter was less than sympathetic since I had always given her a hard time about not eating watermelon.
Surprisingly, after not eating any for some fifteen years, I am no longer allergic to it at all! Go figure! I eat all I want in season when they are cheap, and lay off completely out of season. I don't eat it at all if I am having other allergic problems. So far it works.
Thanks for writing.