I bet you are riding with straight elbows. Keep working to strengthen your core and really focus on your position on the bike while you are riding. Relaxed shoulders, relaxed elbows, relaxed hands. Shrug your shoulders every once in a while, and be light on your hands. Wiggle your fingers. Breathe and smile.
I'm sure the PTs here will have more concrete advice, but do the above and it will help.