Did in my left-index fretting finger a while back - swelled up like yours.
All I can add to the above is that I followed a course out of a book called "Treat your own Injuries and Strains" which basically involved applying ice and little stretching exercises every four hours followed by more ice, and complete rest of the affected area. Any pain, you should stop immediately what you're doing - adding injury onto injury. Stretching the joint is different, shouldn't involve pain.
Took about 3-4 weeks to come right, but I've had no further trouble - or rather, I can feel the limits of the ligament now and don't overdo the 12-string. The overall advice in this book was unless you heal these things properly they become recurring injuries and they will flare up regularly.