U dint mention mention whether u sustained any injury or not.Reason could be due to bony or muscular inflammation due to long hours of sitting
Next Steps
avoid sitting on hard surface. sit on special kind of pillow called coccydynia pillow or doughnut pillow. it is a round a pillow with hole in between. sit on that only whenever u sit
Health Tips
take Tab. AKILOS-SP 2 times a day for 4-5 days... still if u don't get relief then consult orthopaedician either by video call or in person