We are all such worry worts - me included! You get so used to the amount of movement your bub does so that when there is a sudden change you stress!!! I had this a few weeks back - bubs hardly moved for about 3 days and I was in Greece so there isn't much I could do about it... anyway he started again - back to his usual form!

I have also read that when they run out of space they can't kick as much...

You should trust your instinct though and if after a while you are still stressed get it checked out.