First try an O/C antihistamine like BenaDryl. Antihistamines are used to treat skin conditions and reduce rashes. You may have some type of bug bite especially if the rash is concentrated around the waistline. They tend to gather in areas that are bound or restricted like the waist, groin, top of the calf from socks or under the arms. If the Benadryl does not work it would be best to be seen by your GP. Good Luck.
Does the rash resemble this: