OMG, girl, you are NOT being selfish. The people asking you to "pump and come" are being selfish. Even if they don't realize it now, they will when it's their turn ;).
I have the same issue. For us, one of the weddings is the week we are due, and the other is a moth after. We are declining on both. The one a moth after delivery is about a 10 hour car ride, or a very expensive plane ride, and
the bride and groom both understand why we can't make it.
Sometimes you've gotta make the tough call, you know?