Do you walk a lot? There is a lower leg rash known as ‘golfers’ or ‘Disney’ rash, which is a type of vasculitis, but usually appears in hot weather, and in older people. If that is what it is, it usually clears up after a few days. I have had it myself in the past. In any case, might be wise to contact your surgery, and send a photo, so your GP can advise you.