From your description - a single small painless pimple just above the anus, normal regular stools, no pain, bleeding or discharge - this is most likely a simple infected hair follicle (folliculitis) or a small sebaceous/blocked gland in that skin. That area has coarse hair, stays warm and moist, and gets easily irritated by a hard jet spray, tight or synthetic underwear, sweating and friction, so folliculitis there is very common and not related to your bowel habit. Since it is painless and you have no fever or swelling, it is not behaving like an abscess or a fistula at this moment. Most such pimples settle on their own in about 1-2 weeks with simple local care, and yes, it can often heal without any oral medicine.
Next Steps
1) Wash twice daily with plain water and mild soap, pat completely dry, and apply warm compresses for 10 minutes 2-3 times a day. 2) Do not squeeze or prick it, and stop using the hard jet spray at high pressure. 3) Wear loose cotton underwear and avoid shaving that area till it heals. 4) A simple antiseptic like povidone-iodine can be dabbed on; a topical antibiotic cream may be added after an examination. Oral antibiotics are not needed for a single painless pimple. 5) No special diet is needed, but go back to simple home-cooked food, plenty of water and curd after 10 days of eating outside. 6) If it is not gone in 10-14 days or it keeps coming back in the same spot, get examined once. You are welcome to share the photos with me on a Practo online consult, or get examined at Synergy Multispeciality Hospital, Miraj.
Health Tips
See a doctor the same day if it becomes painful, red and hard, starts swelling rapidly, discharges pus or blood, or you develop fever or difficulty sitting - that would suggest an abscess, which needs prompt drainage and should not be managed at home.