welcome to hcm forum
as per your history you had small accident, i think that during the accident you might have fallen on your buutocks and have injured your tail bone.. so i advised you to get a xray of the coccyx
bone and with report meet an orthopaedics. For the time being i will advised you to take rest analgesics and use a hollow/coccygeal ring(ring with hollow in the centre) whenever your sitting..
wishing you early recovery