My guess is they are tapeworms. However if your one dog is losing weight she needs to be seen by a vet. Have you tried this for your flea infestation:Amazon.com : Bayer Seresto Flea and Tick Collar, Large Dog : Pet Flea And Tick Collars : Pet Supplies
Here is information on tapeworm. http://pets.webmd.com/dogs/dog-tapeworms-symptoms-treatments
Here is information on tapeworm. http://pets.webmd.com/dogs/dog-tapeworms-symptoms-treatments