Hello
As you mentioned that these bumps over butts are from last 5 years and sometimes become red and if popped bleed . These bumps may be due to these reasons , these include.
1
Diabetes as this is the most common cause of such type of bumps . Diagnosis can be confirmed by blood sugar level .
2 Due to unhygienic condition some times sweat glands develops into
abscess ( due to infection ) .
Consult a dermatologist and get his opinion as physical examination is important.
3 Frunculosis as this is most common cause of such
boils . Usually staphylococcal or
streptococcus bacteria are most common pathogens responsible for such infection . Get in swab from these boils and get it be examined in lab .
There are so many other reasons as continuous friction due to seating may develop such boils .
Worm infestation is another such cause .
At present 5 years passed so you must consult a surgeon and get his opinion as physical examination is important.
Hope this information will help you.
Good luck.