The girls going out on Tu B’A (and Yom Kippur) were presumably penuyos tohoros (because in those days even single girls were toivel), which our girls are not.
If Chazal cared enough to make the girls borrow clothing not to embarrass those who did not have, Kal V’Chomer they were careful not to embarrass the girls who were Niddos and identify them to their peers and townspeople.