Large muscle in the back of my leg below da knee has been stiff and painful for days. i don't remember an injury now spasms after wo. treatment?

Sprain: Most likely you sprained your gastrocnemius (calf muscle) while working out. This can happen with exertion or overuse, and not just from a traumatic event. Your calf is probably trying to "tell you" in its own way to rest the muscle while it heals. I often recommend RICE therapy for this:Rest, Ice (20 min at a time) compression (ace bandage not too tight!) and elevation, however rest is the key!