My mother has pain in her thumb joint for more than 2 months. its difficult to bear weight from that hand also. she once got removed a tumor from knee 4 years back. is there any link? what type of doctor should we approach? what tests should be done?

Please have her see an orthopedist. Start with general orthopedist or a hand specialist. Possible link but likely different causes. She needs an exam, possible X-rays, possibly MRI.

the joint at the base of the thumb is the most common site for symptomatic osteoarthritis. She should see a hand surgeon for evaluation and possible xray. In the meantime a wrist based thumb splint will offload the joint and decrease her discomfort.