What might this be? an itchy rash like cluster of bumps on my lower back just above my buttocks. it looks a lot like a bunch of mosquito bites.

Flee bites?: A cluster of rash on the could be from flee bites, if you have pets or if you sat on a grass field. If it is limited to one side, you may rule out shingles. Poison ivy type of contact allergy should also be considered.

How long?: If it lasts over 6 weeks, see an allergist. If it is short term, try some otc antihistamine up to 4 x the regular frequency but watch out for drowsiness. Otherwise topical Hydrocortisone cream should be tried. If it still did not work, see an allergist.