Red splotchy patches on legs that come and go for a couple of weeks now. is it an allergy?

Unlikely, but: More info is needed. If the patches are multicolored and itchy, it could be erythema multiforme; the patches are red, swollen, and itchy, which can be an allergic reaction or else, a manifestation of an infection such as valley fever. Recommend going to a physician as soon as possible!

Can't say: These lesions would need to be examined, a history obtained & other tests may be necessary in order to diagnose them. Usually the lesions needs to be identified to determine the solution.