What happened if you just washed your hair and parts of your hair weave got very matted?

It depends how you washed it, what you used, and how you dried it. It probably matted itself while you were washing it, especially if you were washing it with some sort of soap or shampoo, or while it was drying if it wasn't lying flat. You might want to try gently combing it if it is really bad you could buy a new one.