Gassy baby?! Please help

Natalee
Im breast feeding my baby boy 3wks old, when we go out,I forget to pump sometimes . so I decided to buy enfamil newborn formula...at first when breast feeding he pooped everyday yellow seedy like fart poops...now he's just passing gas and its been two days since he poop. Is it the enfamil? I believe it is but will he get use to the formula or should I stop ?