I would encourage you to get it looked at by an orthopedic or spine doctor, rather than wait. The knot could be swelling or scar tissue --hard to say but at least an x-ray is in order maybe an MRI.
I had a horse for 12 years and took more than one fall landing on my back more than once. Many years later I finally got a diagnosis of spondylolithesis and pars defects (fractures), plus other misc things. My point is: would I have stopped riding? No probably not, but I would have been more AWARE of risks and possibly more cautious. I don't even know if the falls caused the end result of where I am at now, but certainly could have contributed in a cumulative way.
If you have insurance, I would get a diagnosis to see if any damage was done to your vertebra or disks and put your mind at ease.
Hope this is helpful and feel better.
P.S. Ice is real good for pain and swelling. I swear by it. 20-30 minutes only.