Making poopies refers to the act of taking a dump or taking a shit. This is the process of emitting fecal matter out of one's posterior orifice. I don't know why the term: 'taking a shit' caught on, since you're actually 'leaving' a shit'. Making poopies can be a means of describing your current situation or that of someone or something else. This expression can be used in differing tense's as given in the examples below.
Honey, you're not gonna believe this, but I have to make poopies.
Hey Louie, where's the poker dealer? Oh, he's in the shitter out back making poopies
Hey everybody, I just made poopies in my pants!