Baby did something strange today...

I'm 33 weeks, FTM and today my baby did something twice that kinda weirded me out...I felt a light thumping and could see it on the outside of my belly, but it was really really fast, less than a second apart. This lasted for about 20 seconds and then she went back to kicking and squirming, but then it happend again about 5 minutes later. It was almost like it was her heart beat I was feeling and seeing but I don't know how that could be possible...has anyone else experienced anything like this or know what it might be? Thanks!