In most of us, most of the time, it comes down to dehydration and electrolyte depletion.
Just make sure you're adequately hydrated before training. And that you're taking in salt, potassium, magnesium and calcium (via food sources or supplementation). Because when these nutrients fall to certain levels the incidents of muscle spasms or cramping, increases.
Poor conditioning, muscle fatigue and doing a new activity are other common reasons for muscle spasms or cramping.
See how the forearm is tomorrow, you should have no pain or tenderness I hope.
If this is the case, ensure you drink enough water
before you train next time, so that your body is adequately hydrated.
Obviously if any pain and tenderness is ongoing and constant, you need to go and see your Dr.
But I'd bet 100 bucks, you just need a little more water
Be interested to hear how you go! Good luck.