Just in general, it sounds as if the edges of your fabric stretched as you worked. Heavily starching the fabric before cutting helps prevent this kind of distortion, as does working very carefully. Edges cut on the straight-grain of the fabric have more stability. As soon as cuts are not on the straight-grain, they are bias and highly likely to stretch and distort. You mentioned a circle in the middle of your tree skirt. A circle is always going to be on the bias in some way, even if made in pieces.
Posting a photo would definitely help with figuring out your best course of action at this point.