Hi, I can understand your concern for fluid filled blister on buttocks and inner thighs. In your query you have not mentioned your age, sex and other relevant details so that the exact cause could be found out. There can be multiple possibilities of the blisters in your condition. It can be a part of simple folliculitis by bacterial proliferation or may be part of some autoimmune problem which is causing blistering. A detailed history with proper work up is required to reach to the exact diagnosis. You should meet dermatologist at the earliest so that exact diagnosis can be made. Do not try to pinch or pop the blister as can get secondarily infected and further complications. Hope these information will relax you. Take care.