I kind of think she started cramping, didn't really realize she was having a miscarriage, ended up having it at the mall (or nearby) maybe. Got scared since no one (seemed) to know she was pregnant and didn't want anyone to find out, so she put it in the bag til she could figure out how to dispose of it.
I'm going to guess that in her not so smart mind, going to the hospital with it or after it, wasn't an option because again, she didn't want people to know. Because of course the hospital would have tried to contact her parents/caregiver.
So her (also not smart friend) says "okay that's over with. while we're here, let me go get some non-supportive bras, a couple of thongs, and some Love Spell. maybe even some of them yoga pants."
that's as much of a benefit of the doubt I can give this young girl.