The milk inside could be very hot. As you feed, keep the bottle tilted so that the teat is always full of milk. Otherwise your baby will be taking in air. If the teat becomes flattened pull gently on the bottle to release the vacuum. If the teat becomes blocked, replace it with another sterile teat.
Teats do come in different shapes and with different hole sizes.
Find the one that suits your baby. After the feed Gently rub or pat your baby's back for a while to see whether there is any wind to come up. Make sure your hands are absolutely clean. Boil some water in the kettle and let it cool. Take a sterilised bottle and teat.