Thanks for the detailed info
Some of the possibilities are discussed below.
1-The exact cause of thumb arthritis, as with osteoarthritis in general is not known,most of the time this is due to aging,overweight patients,muscle weakness can cause these symptoms.
Treatment is focused on two things
a-Splints which decrease pain, encourage proper positioning and give your joint some much-needed rest. Depending on your needs, you may wear a splint overnight or throughout the day.
b-Medications like NSAIDs for the pain(In case there would have been no contraindication as in your case)
2-The other possibility which comes to my mind is De Quervain's tenosynovitis,in which the sheath of the tendons on the thumb side of your wrist becomes inflamed or swollen, restricting the tendons' movement.
Treatment if started early should result in improvement in 4-6 weeks and includes as follows.
Immobilization same as splint method i described above.
Minimize repeated thumb movements whenever possible,
Heat or ice therapy is better AVOIDED.
Now in your cases like yours there is likelihood that you will get benefit from local corticosteroid injection,it will subside the tendinitis and bursitis around the involved joint and will help you alot.
I hope the info helps.