I started having groin muscle pain after my runs, now. This was true for pregnancy number 1, too, though I don't remember when it started.
I can't remember how or if I fixed it, though, so I consulted doctor Google. I ended up in a Runner's World article about running while pregnant. It was much more encouraging than I expected.
The pain is normal, and cutting back to half speed and distance is normal too. Because of the work your body is already doing, you end up at the same fitness level with half the exercise.
So says Runner's World, anyway. And I'm inclined to believe them. ;)