Barb:
It's tough to diagnose a rash like the one you describe without seeing it. The red marks sound like hives, and there is one form of hive wich comes under bathing suits off the Florida coast. Erroneously called, "sea lice," it's caused by jellyfish larvae. But that stays put, and doesn't move around. In any case, it goes away in time.
Most hives come from nothing at all -- not soaps, detergents, food, stress, etc. My advice is to ask the doctor who already saw and treated you what he or she thinks. If the answer is, "Hives, cause unknown," you may have to accept that. Just because a rash has no cause doesn't mean it can't be treated and made to go away.
Best.
Dr. Rockoff