She could have been innocently and coincidently looking in the same direction but if it was longer than what could be reasonably ruled out as a "coincidence" then dude, no need to even ask: she was wondering how big your stuff is, what it looks like, and if she's into you then she was imagining the things she'd like to do with it or have you do.
Haha, girls think more like boys when it comes to sex than they let on.