The good news is that as an organist you don't have to worry too much about being able to play keys with different levels of force like pianists. In fact the organist at a Church I played at switched from piano to organ specifically because his Arthritis was less troublesome when playing organ.
When my hands were a bit weak as a result of a long term debilitating illness, I recovered strength and flexibility in them by a mixture of coin tricks, card tricks, and a set of squeezy balls of incrementally increasing resistance. Mileage may vary, but that did help me.
It may be worth checking your ergonomic position and posture at the keyboard, including back, shoulders, arms, and hand positions, to ensure you are playing free of unnecessary tension.
These things said however, I suspect the best advice anyone could give in the circumstances would be to urge you to consult a professional (doctor or physio) to have your hand looked at, as they could examine and see if there's an underlying problem to address, and also be able to offer advice from a medically trained perspective.