How serious is this tailbone injury?

About three years ago, I fell off a tree stump that was placed on concrete and landed hard on my butt. My tailbone hurt really bad for the next few hours, but the pain eased up, so I didn't go to the doctor. However, since then, sit-ups have been horrrible. I feel my tailbone in the way, and afterwards, my tailbone has bruises and is sore. Today, we did PT at school that involved doing 30 sit-ups. My tailbone has hurt all day, so I checked it when I got home, and the skin around it has been rubbed off. Is this issue serious enough to have to go to the doctor, or is there an easy way to get around it?