Not sure what this is and so not sure how best to treat it - any advice would be gratefully received.

For some months now, I seem to be getting pain in the ankle area which is hard to pinpoint and doesn't seem to be easily located with the odd prod or two. I would say it is to the rear or side of the achilles and the achilles tendon itself does not seem inflamed, though is probably tighter than it should be. I can often do short runs without pain and it seems to be going down hill that brings it on. Day after a run, it can be sore even if it didn't hurt at the time.
The usual RICE treatment eleviates it for a while but it returns next time I hit the hills. Have rested completely and tried running on the flat to no avail.
Have not had new shoes or anything else that I can think may have triggered this.

Any ideas?