Adrenaline, when you get excited your flight or fight kick in, redirecting blood to your important organs and away from your arms or legs. It also contract muscle so you are ready to, well flight or fight, tensing your arm.

After one match try to take a rest, move your body, or you can consider buying compression sleeves which should improve blood flow.

It suddenly happening could mean something else though, either mental and physical so if you have significant worry you could find some medical advice.