Well, I'm no expert, but perhaps the baby's moved to a different position?
I also agree with Emma, I might not feel much movement during a given day, only to wake up the next morning and have DH tell me that he had his hand on my belly while I was asleep and the baby was kicking then!

But you are the mum, go with your gut instinct, if you feel something is truly wrong then call your ob/midwife. All the best