A saved womn can do whatever she likes. But if she wants to glorify God in all things and at all times, and if she wants to show forth the biblical distinction between men and women, and if she wants to wear only modest apparel as the Bible says, then she won't go around in sweatpants. After all, sweatpants are only a form of pants, which are men's apparel.
Please think about this. For centuries women managed to do all kinds of activities wearing dresses. Let's not rationalize our desire compromise.