I might still stick with the algae theory tho. If something gets trapped in your bathing suit, it has to be real small to get in there and get trapped, typically around constricted spots. I have a niece that seems to attract jellyfish, and her stings have all been on exposed areas and have a traditional branch or zig-zag set of localized welts, not rashes.

Been there, done that, the washing machine ate the T-shirt