Hello. my thumb got bent backwards and i heard a cracking sound. and it hurts but i can still move it. what could this be please?

Thumb sprain: Hi, the cracking sound you heard is more than likely fluid between your joints. If there was no other trauma besides the thumb bending backward, it is more than likely a sprain and not a fracture. If your symptoms worsen and persist for more than a few days, it would be advisable to follow up with your doctor.

Possible tear: The most common injury to the thumb is a ulnar collateral ligament tear known as gamekeeper. My advice would be to go see a doctor and get a good exam and imaging: xray which could show a bony avulsion. MRI should also be considered to appreciate the ligaments.