When Carolina had hookworm, she started out with diarrhea. I fasted her for a day and then started a bland diet. Even with the bland diet and smaller more frequent meals over several days she still had pudding poo.
Carolina and Jake are on Heartguard Plus year round, and she still managed to pick up hookworms. Take in a stool sample and have it tested.
ETA: Jake was treated too, just to be on the safe side.