Large lump on right upper beginning of butt crack. painful sitting. red and swollen, not sure if its bone or just swollen. what could this possible be?

Cyst: This could be an infected pilonidal cyst. Have a surgeon examine you for diagnosis and treatment. In the meantime, apply warm applications until the examination.

Abcess: It could be an abcess that needs antibiotics and possibly incision and drainage.

Infection: Sounds like a pilonidal cyst that become infected, and if so, may need incision, drainage and antibiotics.