Definitely a side effect of some of the AIs, not just Letrozole @Afraser ..... I still get it now & then - but it seems to 'cycle' - bad for a few days to a week & then 'fixes up' for many months. I used a thumb splint too - I am sure it helped. It was the weakness in the hand that really annoyed me tho, not being able to hold things (even light ones!) Prednisone helped when mine was at it's worst too - but mine was very painful too. You are lucky if there is no pain attached to it.
Mind you - I WAS taking the magic oil a while back ..... I had a break from it in recent months & will be getting back onto it again, as the hands went achy/tingly & the general body aches returned too ....