Hand flexer tightness

This happens infrequently, but when I pull down the tendons in my forearm tighten up and keeps my fingers from moving. My fingers and palm get locked in a position as if I was on a sloper, with my thumb ticked under. It is extremely uncomfortable and I have to massage the base of my palm to work the tendons out and regain movement. If I try to pull again the tendons seize up again and I have to repeat the process, and I have to continue massaging for about 29 minutes before my forearm feels normal, without any tightness. I don't know anyone else who has had this problem, so I don't know anything about it. Any possible symptoms or preventative measures you guys could suggest would be greatly appreciated.