In reply to haworthjim:
Likely De quervains tenosynovitis if near base of thumb , and yes , tendon / tendon sheath inflammation likely.
Sometimes a simple soft splint helps , analgesics are up to you , ibuprofen for me as it’s probably inflammation and it’s an anti inflammatory!
Pop in to your MIU or GP. You don’t need x rays.
Gets better in a few weeks .
Good luck