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

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.
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!