What Causes Recurring Blisters On The Buttocks?

HI
please advise on this rather ebmbarresing problem.if i sit on my bum with my legs outstretchd in front of me on a bed or floor or basically anywhere, i develope these little pimples /blsters on my butt..they eventually grow bigger adn then burst and bleed.i have had soo many and they have now left ugly marks on my bum.please advise as to what it could be and what can i do to prevent them and also what can i do for the marks.thank you soo much..