hi there, usually a fall on the bottom causes injury
to the coccygeal bone which explains tha pain in your back. You may get an x-ray taken tho' there is no definitive treatment for coccygeal fractures
. You could use painkillers and may use a doughnut pillow or a cushion to sit on till the acute injury subsides. Internal damage is unlikely but you may visit the ER to get yourself evaluted to rule out any internal injury. Take care.