“knot” on inner bottom of heal

Hi,
2 nights ago, while playing cards I began to feel a pain in my left foot and realized I had been resting my foot on the rungs of the chair (without shoes on) and put them flat on the floot.
Later, when I went to stand I felt like I was walking on a knot on the underside of my foot.
The knot has not gone away, I have tried massaging it, soaking it, etc.. nothing is helping.
When I put pressure on it, it is slightly painful, but more of a nuisance than anything.
Could this be caused by the pressure of the chair rungs, or is this something else?
What can I do.. is it possibly a blood clot?
Thank You, Angie