I have a rash with blisters growing on my leg without any cause?

Possibly infectious. If the area is painful and/or there was pain in the area before the blisters appeared it could be shingles. They usually are pretty localized and the blisters will crust over. If it is recognized early there are medicines to take but even without treatment will usually resolve on its own but may leave residual pain at site. Your primary is the best place to go to be evaluated.