I helped one of my in-laws move house a bit back, and I either fucked my back then or when I stood up and sat down ten times while exercising; I'd say the lifting wasn't good, and the exercise a week later topped it off. I don't know; it was weeks ago, and I'm fuzzy on events.
I feel guilty just knowing that the next person who asks me to lift something, I should say no, but it's hard to say no when people need a hand lifting something.