For whatever reason, you are starting to bear weight on either the tailbone itself or on the ligaments that attach to the tailbone.
Your saddle might just have broken down so that you are sinking in too far, and as you sink in the saddle padding presses upwards and jams into your tailbone stuff.
I'm thinking this is more likely than that the saddle itself is the wrong size. If it were too narrow you'd've been suffering long before this.
Can you get a new version of the saddle? Since you liked it before this tailbone pain started, it might just be a case of it having worn out after 1,500 miles.
If you suspect that maybe the saddle doesn't fit, it's going to be the width that matters. Tailbone rebound pain often happens on saddles that are too padded or too narrow. Measure your sit bones and make sure the saddle is at least as wide as the outside width of your bones. Even if a saddle is only slightly too narrow, people can ride on them: they'll perch one bone on the saddle and the other just hangs out until they shift around, then suddenly they are jamming the saddle into their tailbone stuff.
Wouldn't hurt to measure your sits and the saddle before you buy a new one, but if you only recently started having tailbone pain I would guess it's just that the saddle has broken down. Kind of like how running shoes break down and start hurting peoples' feet after months of happy running.
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