It’s a local “myth” here in Iran that eating watermelon and milk (or any fatty food in general) is bad (as in it can cause stomachache or a bloated stomach etc). How true is this?
My web searches around suggest this myth also exists in India, and there are anecdotal stories on both sides.
For example, see Here's Why Ayurveda Says You Should Avoid Drinking Milk After Having Watermelon.