I am 49 years old, in pretty good shape, I took a very hard fall on my left hip and butt.  The bruise covered my entire left side and thigh, I had no real pain other then the area of the bruise itself,  but after 10 days or so bruise slowly going away my lower back started to hurt.  Rested did all the right things still crazy hurts.  Cant sleep at night. having hard time working now. going on for four weeks now.
It looks like that you may have hurt your back very badly. Sometimes even muscular injury can cause pain up to 3 weeks. Are you feeling like worsening than I would recommend you to get an X-ray of your spine? Sometime even such trivial fall can cause vertebral fracture, so ho harm in getting an x-ray. I would also recommend you to take muscle relaxant such as moral, apply local diclofenac gel and take warm shower.