my tailbone, shoulderblade tips and knees will seriously actually get bruised if i sleep on them too much; i take care not to fall asleep on my back (protects the butt + shoulderblades), and i honestly stuff a pillow between my knees, so the bones don't bruise. i'm not even that skinny at all, but bones are new territory i guess it can be painful to NOT be fat also...
oh, and do you do yoga / pilates by any chance? pilates without a THICK mat will aggravate my tailbone in like .05 seconds