I have a cluster of blister on buttocks (crack)and never had it before. They look they are water filled or colored blisters. I also had fever and body pain. Can u tell me what it could be?

Don't want to guess. This can't be diagnosed with the information that is available. A history, physical examination & other studies may be needed to determine cause/causes. Once accurately assessed a treatment plan can be developed.
See doctor. and have it checked and diagnosed then it can be treated. Could be herpes, shingles,bug bites, contact dermatitis.....