I Have Been Having Lower Left Back Pain Since Last

I have been having lower left back pain since last Friday. It hurts worse when I'm setting. Standing and bending are fine. Twisting is a BIG no! Is this a strain? Symptoms came on after a night of bowling and then setting side ways in the back seat of a truck I was selling. Walked in to the house and BAM!!! Instant pain. Any idea what I am looking at? Laying on my stomach and walking makes it feel better.