As others have pointed out, I agree it may be time to let your daughter know... before she starts forming opinions and speculating on what "could be"... namely that you're having an affair. Whether or not she believes that Mom is aware and supportive, it could lead to resentment and anger on your daughter's end.
It's unlikely she would arrive at the conclusion you're a cross dresser. (then again, with the pantyhose wrapper situation, maybe not)
Lorileah's point is well taken.... We expect honesty from our kids at all times and tell them that no matter how hard it is... they need to tell us the truth and that we'll still love them. Do we not owe it to them, when we're asked questions, to give them the same respect in turn? (Obviously taking into account here that a child is old enough and mature enough to understand)
Just my 2 cents.