My German Shepherd, Fleas, and a Nasty Tapeworm?

Ok, so my puppy Marshall whom is 15 weeks has had diarea all day long and I saw a tapeworm come out of his poop today. I am taking him to the vet in the morning because I am concerned about his well being. But anyways, I have heard that tapeworms come from ingesting fleas and the fleas hatching or something in the intestine. The thing is he doesn't have flees, I give him the topical treatment advantix and I bathe him every other week at Petco. I work there as a bather/groomer's istant so I can give him free baths. I use their flea and tick shampoo every time but I honestly have never seen a flea on him. I have looked on him several occasions just in case. And he is an inside dog so if he did have fleas it wouldn't be very many. Do any of you have any suggestions?