My midwife always tells me to ring her or the hospital if I feel my little girl kick less or her movements change & I'm only 26+4weeks, they'll check your babies heartbeat or they'll scan you better to be safe then sorry or try a cold drink & lie on your left side
Call your midwife if you notice a decrease in movement, it's not because the baby has less room to move! We have an organisation here called count the kicks and they say if you notice any change in movement call your midwife and get checked out. I'm sure everything will be fine but it's best to be safe.
The bigger the baby the less room the have to move hold down on the side of your belly until baby move thats what I do when I realize my baby not moving as much