This sounds like you may have pinched a nerve, or have some inflammation in your back. It could also be more serious, like a fractured vertebrae.
Since this pain follows an accident, and only things like x-rays or MRIs can determine for sure what's happened, you should really go to your student health center and get this checked, and skip the swimming and other exercise until you do.
https://www.healthlinkbc.ca/health-topics/bacpn
https://www.mayoclinic.org/diseases-conditions/back-pain/symptoms-causes/syc-20369906