i have not done AS, but I have gotten B 12 and cortozone (spelling?) injections from my doc in my rear and every time there is a hard lump like you speak of. It's just internal bruising ( according to my doc), like scar tissue but not permanant. When you inject IM, small fibers are torn, etc. I know it hurts, but if you rub it firmly it helps big time to break up the lump. Hope that helps.