Run! Like others said, if you've been doing it, there should be no problem. I ran my entire pregnancy. Some days I had some side pains or cramps, but other days were fine. Just listen to your body, if you need to walk then walk. I often walked up hills, and I definitely went slower! I could still run just not nearly as fast. I also wore a back/abdominal brace while running. It really helped the "bouncing" feeling :-). I ran a half marathon at 18 weeks too. The only reason I stopped running (at 37 weeks) was because I tripped and fell on my belly and my dh banned me from doing any more till baby arrived ;-(. All was well though, just had to spend four hours being monitored at the hospital but it would have been fine since I was full-term.
So go ahead and run if you feel good, just be careful and listen to your body.