When a baby is exclusively breastfed, her poop is generally very mild smelling and may be either liquid or the consistency of cottage cheese. Adding anything else to her diet, such as formula or another supplement, can cause a dramatic change in the odor and appearance of her poop.
If your baby is getting nothing but breast milk and suddenly develops a lot of mucus or blood in her stool and the odor is foul, then it is important to have her checked by her doctor. It is possible that she is reacting to a protein in your diet — such as cow's milk protein — causing her gut to become raw and irritated. It is also possible that she has a bacterial or viral infection that needs further treatment. If she is running a fever or not eating well, then it is important for her to be checked right away. If she's otherwise content, but you're noticing a lot of mucus in her stools, keep track of what you're eating, and see if the mucus subsides with a change in your diet (like eliminating dairy products). If it does not resolve, or it gets worse, then definitely take her to the doctor.