What Causes A Painful Knot On The Heel Of Foot?

About 2 months agains I went to walk in my kitchen that has ceramic flooring. I stepped on the edge of the flooring with my heal and felt something crush. I thought I had stepped on something but after examining my foot there was no hole or blood. A few days later a knot appeared on my heel close to the back edge. It is hard for me to walk on it cause it hurts. While I have my tennis shoes on it doesn't hurt as bad but as soon as I take it off it hurts really bad. Any ideas?? My sister thinks its a corn.