I had this happen throughout my pregnancy and then a few weeks ago i had it happen where it would go on for ages.

It was just the same as you, happened if i was on my side and i noticed it would keep doing it unless i moved, then if i went back onto that side it would do it again.

I asked when i went in for a ctg and they got the OB to check me, she said it was a positional thing and caused by the increased blood flow and she didnt think it was anything sinister but if i kept happening after i gave birth to get it seen too. She also said that if u are a smaller person, u become more aware of ur heartbeat.

Definitely ask about it though, its not worth taking the risk.