I've experienced significant tail bone pain on saddles that are too narrow for me. This is usually also combined with some degree of numbing toes or loss of bloodflow to the sensitive bits.

My theory is that a too narrow saddle causes your body to be suspended by soft tissue over the saddle - and that soft tissue is pulled taut across the tailbone. On shorter rides this might not be noticable - but once you exceed a certain amount of time on the saddle, it can result in a bruise that is noticable for days after, especially when sitting.

My reccomendation is to make sure it's your sit bones being supported on the saddle - and there are tons of posts around here on how to do that. If they are, then all of the above can be ignored

I hope you heal quickly - if you need to do your ride on this seat, get out of the saddle frequently to help keep blood flowing thru that area.