You are responsible for support no matter if there is a support order or not.
Your ex can ask for arrears. It depends on what state you are for how many years back they can go.
The affect all of this has on your daughter will have a lot to do with how you react to this in front of her.
Please do not talk bad about her mother in front of her and PLEASE do not complain about how much the support is in front of her. She is a child and these are adult issues.
If you keep your home warm and inviting, she will not be affected by a move to a smaller house or appartment at all.
Children need food, shelter, love and support. The rest are just "things".
Your ex maybe angry over the custody hearing, but it could have been worse. After all she let you get away without paying support for YOUR child for three years. Then offered you what, in hind sight, was a good visitiation plan and you decided to take her to court anyway. You are lucky that she didn't go after arears as well.

