If it is in the middle and above butt cheeks, then it can be a Pilonidal cyst.
These awful pimples are tough to treat. My doctor said it was for Foliculitis, caused when staph bacteria we all have, finds a hair follicle clogged by sweat (or irritated), and travels down to the root. I only started getting them a couple years ago, here's what works for me. Wash with charcoal or sulfur soap (castile works, too. Use a loofah or a scrubber, don't scrub hard). This exfoliates. Pat dry. Next apply Hibiclens antimicrobial liquid soap to cotton pad and pat affected area, Let dry, then rinse. Pat dry. To pimples dab 10% Benzoil Peroxide (will bleach fabric, be careful) Or Sulfur Cream (stinks so use at night). Leave 10 min then rinse. For large pimples apply cream to bandage. Repeat daily. Tips: Drink lots of water. Hot Sits Baths, sit on a heating pad for 20 minutes (don't burn, wear cotton). Or try ice. Heat and cold help kill bacteria. To reduce swelling try ice pack and cortizone spray. Wash hands before sitting on toilet, and use clorox wipes to clean toilet seat each time. Use Free & Clear detergent, change underwear daily, clean sheets weekly. Act fast when they are small (cream on bandaid), be patient, don't squeeze! Let them drain naturally, try clay mask to draw-out infection when close to surface, hot bath and warm compress to drain gently palpate till clear. Once skin is open use triple antibiotic on bandage till healed. Hope this helps!
I've discovered that the pimples are a result of candida...too much sugar....breads and alcohols consumed....when I cut out sugar and fruit, alcohol, breads etc I don't have the problem...i put a fungal cream on the pimples...yes like Monistat cream to kill off the fungus...candida is fungus in your stomach and since it's alive it craves to be fed...that's why we crave the sugar etc...and most people have lots in their body....