my bb is drinking from avent bottles. I'm feeding fm. is it that when I tightened the bottle too tight, when bb drinks. many tiny bubbles will form? I loosen a bit yet still see bubbles. is it because the teat flow is too slow?

Did you shake the bottle when mixing the formula? If yes, move to gently swirling the water and milk to mix and not shake. And then leave the bottle for 15 minutes to "sit" and get rid of the gas. You can also stir the milk in with a butter knife.