Hello and thanks for the query
In my opinion you must consult your doctor and get clinical examined. It may be a pilonidal sinus
or a abscesses or injury
to tail bone. You needs to get treatment according to the cause.
Do hot fermentation at the site and use analgesic
and anti-inflammatory drugs to reduce pain.
keep on changing the posture while sitting so the pressure cannot be maintained on the site.
Use cushion for sitting.
Hope this will help you.