Dog leash jerked thumb backward when my dog took off. it hurts and certain movements hurt worse, at base of thumb. what happened? what should i do?

X ray: you apparently had a hyperextension injury to the thumb, I suggest you have a medical review to assess the need for X ray on, if there is skin laceration that will need to be looked at and possibly given tetanus vaccine if required

Thumb injury: get an X-ray . get it examined by doctor to make sure there is no scaphoid injury-- unlikely with this mechanism though. make sure no boney injury and then a thumb splint for resting the thumb will be helpful. physiotherapy will be advised.

You have: strained (hopefully not torn) ligaments attached to the muscles responsible for the flexion (bending...grasping) of your thumb. Apply ICE and elevate your hand for the first 24-36 hours...then apply heat.... Do NOT strain the thumb more..REST your hand for that 36 hours. After that if WEAKNESS or PAIN or INABIITY to move the thumb persists ..a Hand Specialist should be consulted! Hope this helps