I am not a runner, and I haven't done spinning, so maybe I'm way off base...
From what SUMMERLVN85 said, it sounds like its a muscle(group) that you don't usually work, that got worked pretty good during the spinning class. Now that muscle is sore, maybe putting pressure on things around it that it wouldn't otherwise. If it feels more like muscle soreness than pain from an injury, it might be a good idea to do another couple spinning classes or something similar (stationary biking?) to continue strengthening that muscle. It seems like once it got strong enough, it was no longer sore for SUMMERLVN85.
If it feels more like pain from an injury though, you might try just taking it easy, stick to walking and maybe easy biking, and let it heal. If it keeps going on, you might consider talking to your doctor, make sure it isn't serious at least.
You might also do some research and see if you can find a stretch or two that stretch the pelvic region of the body. I know whenever I have soreness of any kind, from just tired muscles to pulled muscles, stretching always helps it feel better.
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