Why do our newborns squirm so much after feeding?

My wife and I had our first kids – a twin boy and girl – three weeks ago. They seem perfect in every way but we’re finding feeding them very stressfull. My wife is working on breastfeeding them exclusively but for now that isn’t possible so most of their feeds are formula. A lot of times the babies squirm and pull contorted faces a few minutes after feeding. This goes on for some time and it upsets us until they settle down. We burp them carefully and patiently and don’t lay them straight down.

We have tried Aptimel and SMA formulae. We have tried tepid milk and warm milk. Time-based feeding and demand-feeding. Small quantities and whole bottles (as they demand it). Nothing seems to quite crack the problem. The squirming and crying is ofter accompanied by them throwing up a not-insignificant quantitiy of the milk. Is there anything else we can try?