Hi, could be from hanging on too tight and sitting too upright.
Practice riding with relaxed shoulders, arms and writs. Don't grip the bars too tight. Occasionally relax all your fingers, aim fingers forward and wiggling them. You hands should not have any upper-body weight on them at all, that is, you can take your hands off the bars any time and not have your upper-body tip forward.
That's the other thing with sitting too upright, you bend your wrists and put weight on them. Bend your elbows so the forearms are in-line with the hands so wrists are straight. More aero if your forearms are parallel to ground. This requires bending over, best with straight-back to reduce fatigue. Bend at the hips, rather than middle of back. To keep throttle-wrist straight, "walk" your hand along the grip after you turn it.
Work on back-muscles. Do lots of roman-chair back-lifts at gym; I've gotten to where I hold 30-40lb weight when doing this exercise. Also do lots of stretching.
