Freaking Out!! Please Help!

OK, so I will be 32 weeks on Thursday. For about a month or so, sometimes when my baby moves, it's so rapid, like a vibrating/trembling. I heard one person describe it is a jackhammer and thought that was a perfect description of it. I tried to Google, but obviously didn't get anywhere good. A lot of people are saying it's perfectly normal yet no one knows exactly what it is, while one person said it was the baby wrapped around the umbilical cord and seizing b/c of loss of air supply. Have any of you experienced this feeling, and if so, can you shed some light on information you have? I have an OB Appt. on Thursday and we'll be asking about it at that time, but looking for any information until then. Thank you!