Let me guess-- you have a WICKED ragweed allergy, right?
The timing suggests that it's likely to be IgE-mediated somehow. (Meaning it's allergy related somehow.)
It's sounds like classic
oral allergy syndrome. This is where your immune system mistakes a food for a pollen, basically. It's usually not dangerous, but can be
really aggravating to live with, as you know doubt are all too aware.
Melons are a relatively common OAS offender, and a quite
rare primary allergy. This would be even more highly indicated if this is more severe (or maybe only a problem) at some times of the year-- for ragweed that would be late summer into fall in most places.
My first spouse, though, had a true anaphylactic allergy to watermelon, so it can happen. Good idea to talk to your doc about it to be on the safe side.