This is going to sound awful - I actually deleted it once but I'll put it:
The scientist in me would say (1) see if this wears off as the melon goes through her system, and then... if it all clears up... (2) give her another bit to see if the same thing happens

But I know for sure I wouldn't be able to do (2) myself even if I really wanted to know. It's easier just to avoid melon. However, can you remember if you gave her any melon before when she got poorly?
Also check out this link where people with IC (and I know piggies aren't people) are debating whether it's a trigger:
http://cobf.websitetoolbox.com/post/I'm-telling-you-its-the-watermelon-1956114
If she's got this SIC it would seem there can be multiple triggers and it's just learning what to avoid
Poor Brownie - let's hope it's a one-off...