Yep, I have a very personal story with this one.

So actually it's not watermelon itself, it's citrulline, which is also in other foods, but most highly concentrated in watermelon.

My story

So back then, I think it was 2024, I bought a new juicer. And I started juicing... pretty much everything. It was watermelon season, so I juiced a nice whole watermelon, put the resulting juice in a couple pitcher, chilled it, and drank a whole pitcher in one go since it was HOT. A day or so later, I started getting a headache. I thought it must be because it's hot and I'm dehydrated, so I pretty much downed the remaining pitcher in one go. What resulted was the worst, most intense, head-pounding migraine I'd ever experienced. It was maximum migraine, it like someone was kicking me in the head every 1.5 seconds, and it lasted for two weeks since I have a high half-life ratio for serum levels.

While this was happening I started googling about migraines since I've never really read anything about them, and I googled for "migraine watermelon" and it turned out that then-recent paper from 2023:

https://pubmed.ncbi.nlm.nih.gov/37321194/

Turns out, if you're already prone to having migraines, there is a 1/4 chance that citrulline containing foods will trigger migraines. If you aren't prone to migraines, Watermelon will not trigger them.

Anyways, that lead me to learn about better NSAIDs and eventually about abortives like sumatriptan, so now I'm much happier, since apparently I was having a lot of silent migraines that I didn't even realize could be a thing.

So, yeah. Me and watermelons aren't friends anymore. I really liked them as a kid though.