While the others are probably right and a visit to the doc is in order, but in the meantime, here's a possible simple solution....your saddle is too narrow.

If I ride a saddle that is too narrrow, I get a pain similar to what you describe--just above the tail bone. It is as if someone put in a device and spread everything apart. Standing on the bike becomes almost impossible and I dread intersections. Once I'm up for a while the pain subsides. I discovered this after riding the winter on a good fitting saddle. When I went back to my outdoor bike, the pain started up again. Switched the outdoor bike to the same saddle I was using indoors and voila'--no pain.

Find those threads on measuring the sit bones and then go back and masure your saddle. Do your sit bones have enough room? If so, then look at the shape of the saddle--is it flat across the top or angled to the middle. If the latter, go for the flat.