You have not mentioned on which part of your body you had the rash. Also have you ever been diagnosed with some medical disorder in the past?
There is likelihood that you might have had a mild allergic skin reaction. There is a common skin condition called hives or urticaria, which causes red and sometimes itchy bumps on your skin. Hives can occur anywhere on the body such as the trunk, arms, and legs.
Hives are usually allergic (food, medication and insect stings) but may also be due to infections and stress. Many a times no specific cause for them is ever found.
They usually go away on their own, but sometimes oral antihistaminics such as cetirizine or loratidine (available over the counter) may be required.
If your symptom is still present or bothering you, you should consult a doctor.