You have been provided with an excellent differential. However common things happen commonly, and from what I can deduce you might like to establish the possibility of de Quervain's tenovitis. If the tenderness can be accurately localised to the ulnar side of Lister's tubercle, and Finklestein's test is positive, you justify ultrasonographic demonstration of the combined sheath of EPB and APL. This would be expected to show thickening of the sheath and possibly fluid. If this proves to be the case surgical decompression is far more predictably effective than other forms of treatment (including steroid injections). Ideally it would be performed under regional anaesthetic with a tourniquet (Bier's block using prilocaine), via an 8mm incision. Sutures are usually not necessary. You would be able to use the hand the same day and put the thumb through a range of movements from the moment the tourniquet is removed; keep it dry for four days, and then treat as normal. Please let us know the outcome!