A bump sitting in the natal cleft (between the buttocks) rather than at the anal opening itself, present for months, non-bleeding, with a foreign-body feeling on sitting - that is most often a pilonidal sinus or a simple epidermoid/sebaceous cyst. Cancer in this area does not behave this quietly for months, so stop worrying about tumour. Your examining doctor was right that it is neither piles nor a tumour, but "monitor" is not the endpoint here.
Next Steps
Ask a general surgeon for a simple soft-tissue ultrasound of the swelling - it will tell cyst versus pilonidal sinus versus a fistula tract in one sitting, and stops the endless "monitor it" cycle. If it is a cyst or sinus, small day-care excision cures it. Meanwhile keep the area dry, remove hair around it, avoid long sitting on hard seats. See a doctor the same day if it becomes red, hot, very painful, starts discharging pus or you get fever. Happy to review your photo and report on a Practo consult and tell you if surgery is really needed.