I am a pretty keen runner, endurance etc and had a very similar thing, eventually. I ran (slow, several times a week) for maybe 5 months and then it got really hot and didn't feel quite comfortable (that stitchy pain), so I just let it go. For me, it wasn't quite enjoyable enough to continue with by that point. But I know other people have run right through, so it just depends how you feel.
I ended up just continuing cycling to work etc for exercise, and swimming now and then. Still cycling sedately at 37 weeks no probs. Have missed the running a little, but the mugginess has made swimming pretty nice too. All the best, go with what you feel I guess
This lady wrote a blog about keeping running - it's a nice leisurely read if it's something you're interested in (I think you have to go back to older ones to get the bit where she's pregnant).