Re: Wrong size of the breast shield?
Try lubing your nipples with some olive oil. If the fit is just marginally too small, that may allow the nipple to slide freely and reduce pain. But definitely try a larger size!
Another thought- sometimes a mom finds that she must lean forward into the pump, or the weight of her breasts dragging down causes the loss of a tight seal. So try that, too!
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"