Definitely get the vet back out to have a look. Is it bleeding again? I don't know much about stitches, but he may have to sew it back up. That sounds pretty serious.
Is she in the pasture with other horses all day? You may want to consider stall rest for the next couple weeks because you don't want this to happen again. I know that stall rest causes horses to feel bored, cooped up, and sometimes they get stocked up legs, but I think its more important you keep her from ripping that stitch. If you do put her on stall rest, make sure she always has plenty of hay and water. You could also give her a jolly ball or something to play with.
Yup, I'd call the vet and ask. You definitely don't want anything getting in there. In the meantime, you could wash it with warm water and put an antiseptic on it. And put her in a paddock by herself; a small one so she won't get too rambunctious.
Oh -- WHO KNOWS? [rolls eyes] They can do the most amazing things; you may, in fact it's probable, NEVER find out how she did it. Rubbed up against who knows what, got in a kafuffle with another horse, stretch it out the wrong way (fresh wound, you know), rolled on a hard spot in the ground, banged against a raspberry bush...