Back pain in lower left side of back?

you most likely either slept in a bad position the night before or lifted something heavy improperly. It is most like a strained muscle and I wouldn't worry too much about it. Give it a week or so to heal. Back pain tends to last a while so don't expect it to get better in a few hours. Also, some girls get lower back pain before and during their periods (which could be a possible reason).