This muscular inhibiting effect is used to partially shut down the system to protect you from yourself. If you don't heed its warnings, then it will shut down your system completely. Now, back to your condition involving your back. Your pain is now worse because you have been masking it with the painkillers and continuing to run, etc. It is possible to do irreparable damage to your lower back and knee by taking painkillers and then running.
Painkillers have their time and place, as does the drugless profession of chiropractic.
This is not the time or place for painkillers. Back pain has a number of causes. Bending forward and lifting in the yard may have sprained the ligaments that hold your lower vertebra or pelvic bones together. Most lower back conditions develop from bending over to lift, which causes the joints in your lower back to be at their most unstable point. In this position, the joints are not locked into position tightly as when lifting with back straight and knees bent.