What Causes Skin Rashes?

im 20, 5'3 and 10 1/2 stone.i currently have 2 different types of rashes, the first appeared about a month ago and is a circular blob on my tummy its dry and does not itch, the second is on my forearm and comes and goes but always appears when i have just woken up, any help? are they related?
i have seen a doctor about the rash on my forearm he told me to take antihistamine for a month, the rash went after afew weeks, i stopped the tablets soon after and it has only just appeared again.