In reply to Beardyman:
You won't find a proper diagnosis on a forum (duh) but if that's any help, I had a similar thing a few years ago on my index finger.
The finger would swell, especially after long sessions, and then the swelling would go away after a few hours.
Apparently it was a "bursitis". Nothing serious if compared with tendonitis or pulleys.
I did not stop climbing - I just took it easy for a few weeks, climbing 1 or 2 grades lower and avoiding nasty holds, you know the way...
I hope you find your solution