Breastfeeding question

Caroline

I have been feeding my newborn twins breast milk and topping up with formula as needed as I am not producing enough to only fed breast milk. They are 2 months old now.

As of recently, everytime I feed them my breast milk, they vomit - not spit up but vomit and a lot of it.

As soon as I give them formula- no vomit?

Has anyone been through this or anyone have an idea of what could be going on?