So, in October, I was playing football and I hurt my thumb. It bent all the way back, and since then its hurt to bend it. It also cracks often, and pops constantly. How can I tell if it's sprained, jammed or broken and when should I get it checked out?
You may need to see your doctor with regard to referring you to have the thumb xrayed now at the hospital.
It is highly likely that you may have broken the scaphoid bone in the base of the thumb. After such a long time any bones and sprains should have healed by now.
If things are still bothering you, you need to make an appointment to see your doctor.