posterior forearm pain

Hi doc, lately when I'm training in the boulder gym I feel a sharp forearm pain in the posterior muscles. The pain does not subside after climbing, only when I put a lot of weight on my flexed arm (overhanging). The pain feels sharp and I can't really tell if it's coming from the extensor muscles or from something more in the inside.  This only happens to me when i'm pushing the grade in bouldering. In regular face climbing I don't feel a thing. And after a work session I don't feel any pain at all, just the regular forearm weakness and tiredness. The problem here is that I don't really know how to strenghten or care the injured or weakened area of the arm that is giving me this problem. Thanks!