Apparently from your description, I think you have RSI (Repetitive Stress Injury) superimposed on RA.

The cause is simple .. Your use of the mouse. I guess you use that finger for right click or the scroll wheel. So that kinda work for a few years can lead to RSI especially if you are having RA which causes premature arthritis.
Its showing up in only one joint now but may involve other joints too. So I suggest you definitely get screened for RA.
The test is called 'RA Factor' and you can also do an 'HLA typing' test for the same to be more specific.
I havent seen your images, but from your description I think this is the best diagnosis that i can offer.

Dr Chris.