My 18 Year Old Son Has A Rash / Hives

My 18 year old son has a rash / hives for the past 3 days. It goes away with Benadryl but comes back in a different place the following day. It itches He is not on any meds or had any new foods. Should we see a dr. Or will it run its course. A few days prior to the rash he did have diahrhea and lots of mucus for several days. Could this be related.