Ok, I had a rash like this, and someone related had another type of one. However based on what you have said about the weather around where you are I am going to guess this is Allergic urticaria. Sometimes you can never trace what created the problem, I could not but if you have tryied to eat or buy something new, I would advise to don't use the item again. Also have you taken any of these "dextroamphetamine, aspirin, penicillin, clotrimazole, sulfonamides and anticonvulsants." However we will need to know more about the look of the rash. Hives [the common name for Urticaria] can take many forms, however it is most likly going to look like the skin is raised. The main cure for this is Antihistamine, a small tablet from your doctor. if you have no other symtons I can state it is best to wait, however you should do the glass test just to make sure it is not Meningitis.

How do I do the glass test?
Press the side of a clear drinking glass firmly against the rash. You are looking to see whether the rash fades and loses colour under pressure. If it doesn’t change colour and go pale, contact your doctor immediately