Thanks for input folks; two days have passed since I posted this and for the biceps and left forearm I agree it must have been DOMS. But the right forearm is still sore. if I extend it out, straighten the elbow far as it will go, the strip of muscle on top of the arm from the elbow to the wrist hurts. I can hold the stretch today without too much discomfort - previously it hurt badly enough I bent the elbow quickly, worried I was injuring something worse. So it's improving. The trainer who was showing me TRX may be at the fitness center today - if he is I will talk to him about it.