Yes, being a new mother, if she wasn’t properly taught to change one, maybe she was doing it wrong. I just don’t take the giant leap in logic that she didn’t change it that you do.
And I'm assuming that after three weeks she would have figured out how to slap a diaper on the kid. It isn't rocket science. Fold it into a triangle, tuck once tip between the kids legs, wrap the other two around his waist, pin securely. How do you screw that up?