This is common during exercise, particularly during walking, and especially so when it gets hot and humid. Fluid tends to accumulate in the extremities as the result of the arm swing. It can be bad enough that people need to remove their rings before exercising.
One solution is that every few minutes raise your hands over your head and clench and relax your hands. Some people will carry a squeeze ball, which keeps fluids flowing in and out of the hands.
If it continues after you try these things, then you need to speak with your doctor to rule out anything medical.
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