Spitting up breastmilk, not formula?

Help!

Our little one is four weeks old. I pump and we have fed breastmilk since day 4. We have needed to supplement as well so he has gotten formula for three or four evening/overnight feeds.

Recently, he has started spitting up a lot! Like three or four times after a feeding. However it's almost only when we feed breastmilk!?

We do use different bottles, just for ease because I use Kiinde for pumping, but burping is all the same and we keep him upright.

We use Earthsbest Organic Formula, just the regular kind, so there is dairy and such in it.

I'm wondering what else is a common problem food that I should try avoiding?!

Any ideas!?

UPDATE

thank you guys sooo much!

Swapping bottles and slowing him down helped tremendously! We went from 3oz breastmilk and 4 spit ups in the morning to 6oz breastmilk and no spit ups in the afternoon! 😊😊😊