Niagara Falls does mean a waterfall or city. But it also means a girls period. It is tycially used for girls to talk about among each other without others knowing. It is a nickname for period because for some it might feel like Niagara Falls is coming out of them. Note that you don't say the word period. Just Niagra falls.
"UGHHH that assembly was terrible"
"I know right and I'm on Niagara Falls (period)! So when I finally stood up-"