Hi there - The video of the throw is pretty impressive; my thought is that you have strained a small muscle in your lower back; there are all sorts of short little muscles that help support the spine and move it through space; a strain to a small group of these paraspinals could go unnoticed during the off season, but when called upon to lengthen and support the spine during sporting activities the pain returns; it's treated just like any other strained muscle in the body, i would see if you can have someone identify the point and work it manually with massage and stretch; if you notice improvement then a progression of exercises to get you to full bowling speed would be indicated; a trip to your local physio would likely get you going in the right direction. Regards.