I'm currently using Advantage, Frontline stopped working a while age so I switched and have not have had any problems once I got rid of the house flea infestation.
Your house is most likely full of fleas and their eggs. I used Boric acid (sprinkled on the carpet and vaccumed up the next day)
at the time and still swear by it. But check with your Vet. before trying it.
Its been a few years since my flea problem. Here is a link to my flea adventure: http://www.pets.ca/forum/showthread.php?t=66734
As far as not pooping it is probably just her lack of appetite due to the misery of fleas. If it is constipation you could try putting a dab of Vasline on one of her front paws. About the size of a pencil eraser will do. Repeat after a few days but no more than twice without a Vet visit.
ps. The hotter the house the faster fleas breed, above 90 or so and they will be uncontrollable.