She's had a pad on for a couple of days. Unless she's an evil genius who was planning ahead so she could fool us for some reason I can't quite fathom, she's having her period.
I don't doubt that they were messing around outside in some way. Being on her period wouldn't completely prevent that. But to think that the pad is some sort of cover up is a nutty conspiracy theory.
So now she's wearing a pad just to fool viewers into thinking she's having her period? Because... the script calls for it? That kind of thinking cracks me up ;). I kinda of prefer the simpler option again: that she's having her period!