Welcome to the forum.
The knot is likely to be due to a hematoma
formation in the area.
Since the bony injuries have been ruled out, a soft tissue injury
is likely. You need to avoid massaging the area and take a mild pain killer for relief.
The pain and swelling might take over a week to subside. In case there is an acute pain and a sudden increase in swelling then consult your physician for a direct examination to rule out any infection in the area.
I hope this information has been both informative and helpful for you.