Thumb Pain

I've been getting some pretty chronic pain in my left thumb joint (where it meets the hand)....been going on for a couple of months and it's been diagnosed as tendonitis. I'm guessing the most likely culprit is my bike, but I've had the same road bike for 5 years, over 20k miles, and it's always been super comfy, even on 1000k rides (630 miles in 2.5 days). No setup changes either.

Any ideas? This is really frustrating
Thanks!