Oh boy… poor baby, she has a lot going on!!!
First of all YES going to heat can cause her to poop a lot, it may even become loose. This is to be expected, and yes her vulva will become swollen and red. She also may sleep more and become irritable and cranky… Antibiotics can make her poop more as well. I would make sure to give her a probiotic (we use fat-free plain greek yogurt). You can give her pure pumpkin to help firm things up, NOT the pie filling. I would give her just a teaspoon at first to see if her stomach can handle it. Did they give you anything to take care of the fleas?!! We use Frontline or Advantix to deal w/fleas in our area.