Have you tried lying down somewhere quiet for a bit (15 minutes or so) and drinking a cold glass of water or orange juice?
I'm sure he/she's just having a rest or you haven't noticed the movements. How many weeks are you? If it's really worrying you then phone your midwife who will be able to reassure you, by getting you in for a listen if necessary .
have you phoned your mw yet for some advice, if you feel something is wrong, they should take a look at you and even you go to the doctors and sopeak to them I am sure most docs have sonicaids in the practises..