I've had the same thing happen before. I'd suggest, if you can, to lay off the heavy bag for a while and focus on building up your grip strength and forearm strength. It doesn't have to be for long but the break from bag work will let any slight injuries heal.

If you decide that a stronger wrist/hand could help, then a wrist roller, plate loaded hammer and grippers would do a ton of good. Stronger wrist and hands will help keep your joints in line when hitting the bag and avoid injuries.