Hi,
What you describe seems like sweat dermatitis or heat rash or solar urticaria.
You should try to keep the area dry and clean. Try some powder to absorb the sweat. You could take oral antihistamine medications and apply calamine lotion at the site of the lesions.
If the symptoms are still persistent, consult your dermatologist and get a proper clinical evaluation done.
Meanwhile, avoid sun exposure and use a medicated sunscreen.
Let us know if you have any doubts and keep us posted on how you are doing.
Good luck.
A good sunscreen should help - I get a itchy rash after being in the sun for that amount of time also - I get it from about April - Mid July and once I am use to the sun it seems to clear up til the next year. I have noticed that a sunscreen with a HIGH SPF works pretty well - You can also try taking a Benadryl before going out in it (IF that does not interfere with your epilepsy meds)