While drinking milk or sleeping?
If while drinking milk check the teat. Might be too big. Also, always slightly slant & lift baby's head while feeding.
If while sleeping, make baby sleep on the side. I don't let my baby (I have 4 kids) sleep on their back. I will swaddle and make them sleep on their sides (with small pillow on their back to hold the position & regularly change sides). They only sleep on their back when they're about 4 mths old