I stepped on a mesquite thorn a few weeks ago. It went threw my flip flop into my heel. I pulled it all out and the puncture site is healed up. But my heel is still very sore. I can't walk normally. When examining my foot, there is no bump or spot that I can identify where something may be festering. I can't even push hard enough to make it hurt. It only hurts when I walk on it. What's going on?