Hey Relle,
I get a similiar sensation sometimes, well it's what I think is the same, but I would get mine with all mighty thumps and kicks aswell. I posted a thread awhile ago, asking if anyone experiences it as I was starting to worry it might be seizers or something. From what I gathered from other womens experiences who went on to deliver normal babies was that it wasn't all that uncommon. So I stopped worrying and decided I'd ask my midwife next appointment, but of course I forgot, so I'm glad you posted it again and Alan saw it and confirmed it should be nothing to worry about.
Hopefully I'll remember to ask her next visit , just to confirm it.