Fatigue

I've noticed after pulling a couple of 12 hour days that when I get tired, my tailbone starts to hurt. I'm not doing anything that strains my back, but I start to ache there when I feel fatigued.

I was talking to a co-worker and he had shoulder surgery a few years ago. And when he gets tired, his shoulder aches.

I'm not sure what to make of this. Will I be able to strenghten my back enough that this won't happen? Will it last forever or will it go away with time?

I just get frustrated when I have to stop skating because my back hurts and not because I'm physically tired. I hope I can get this figured out.