"Jelly legs" phenomenon. Fucked so good ya can't walk straight. Perfectly normal post climax state. Tho I suspect it's more common in women, yes men experience it too. Won't happen every time, just when it's really good or you overexerted. Wall slides and calf raises if it becomes a problem, or try different positions if you were straining for too long. Me, I like being on my side behind my partner, them laying down. I find I have the best comfort and longest stamina there. Can really take my time with that bull ass, dog ass, boar ass, cow puss, lol. Except female canines -- for those I greatly prefer missionary.