Yeah, the saddle you are using is probably too narrow. If so, your sit bones are resting on the sides of the saddle, and are being pushed apart which is causing the pain. You really haven't ridden enough for your sit bones to be too awful sore. If you go from 0 to 100 miles they can hurt fairly bad, but after a couple weeks of low mileage your sit bones shouldn't hurt enough to prevent you from riding unless something is wrong with your saddle.