Melon is also one of the fruits and veggies that can be tied to oral allergy syndrome, where the person reacts to a fruit or vegetable because of its similarity to a pollen they're allergic to. For some people, it's year round, for some it's only during pollen season that it's a problem. Some people can eat the food cooked or processed in some ways because it changes the protein.
If you don't have an epi, I'd ask your allergist about whether getting one was useful and about having an action plan as to when to use it and other meds. Per our action plan, what you describe for my son would trigger us using the epi; for other kids it wouldn't.