MDs: Why does it hurt to walk?

Starting on Friday, I’ve been experiencing pain when I walk. It’s mostly centered on my tailbone area, but it radiates down the left side of my ass. ** GMRyujin** thinks it may simply be a case of needing new shoes, as mine are somewhat old and holes have appeared on the bottom of the soles. Last night he looked and said that it looked like the area around the tailbone was swollen. We’re going shopping for sneakers later today, but do you all have any ideas on what the problem is?