[Fazc]

I don't know your training history so I'll just throw out some possible causes:

1) Unusual workload. So if this is a new thing that you're attempting, more volume than you'd normally do or more even just different exercises that can cause cramping particularly if done for a lot of volume. Or related to that, if you've had a particularly hard training session which has depleted you this can effect subsequent sessions also.

2) Unusual diet. If for whatever reason you're taking a lot of caffeine or possibly other compounds (viagra causes huge cramps for some people, I'm just saying!), this can effect mineral balance and can also cause you to cramp. Or you may just be prone to cramping, my brother gets a lot of cramps for no real reason.

3) Forearm weakness. Again related to number 1, if this is an unusual workload then the sheer endurance factor will cause cramping.

All of these will pass eventually, just stay consistent.