Sorry, I am afraid I am a bit late here: my colleagues Graham and Gary already diagnosed the condition. Looks indeed like a tendinitis, most likely aggravated by the use of the mouse. A "flat" mouse can help reduce the strain on the hand (the worst ones are the arched mouse like the old Microsoft). Very useful is also the application of a medicated plaster, containing an anti-inflammatory drug (I am sorry, but I don't know the commercial names in USA). If you cannot find them, follow Gary's advice and take drugs like aspirin, ibuprofen or nimesulide by mouth, ideally with a full stomach. Finally, a cicle of treatment with ultrasounds can be really beneficial.
Let's know how the situation evolves.