Stung by something at the beach?

Possible Jelly Fish, however no direct indication, that leaves the possibility of a LOCAL allergic reaction to that part of the body, and your symptoms indicate that. Even 'Bug Bites' are possible, but they get treated the same way Allergic reactions do, all bites indicated are essentially an allergic reaction (aka exaggerated immune response to a foreign substance causing Histamine release). Treat with topical HydroCortisone (OTC cream 1%) the directions should say rub 2-3 x per day and afterwards put a gauze covering over it. Take Benadryl-pills are fine-this can absolutely be safely taken and will replace Benadryl Cream so only one cream needed. If no improvement in a couple of days, or symptoms get worse immediately consult your Doctor. Avoid that activity until you heal. The remedies I'm suggesting are a combination of an Anti-Histamine (Benadryl) and a Topical Steroid (HydroCortisone 1% Cream).