Hello and welcome to JustAnswer.
The bruising you are seeing on your dog's belly may be a sign of a clotting disorder. The causes of clotting disorders can be something the dog is born with (although you probably would have seen signs earlier in her life if this was the case), or it can be secondary to something your dog was exposed to. The most common cause of this type of bleeding in a dog is if they have had access to rat poison. This type of poison makes it so that the blood can't clot, and the result is bleeding into the skin causing ecchymosis (or large bruises) - these are usually not painful. If you are seeing bleeding in the skin, then it may also be going on internally (i.e. bleeding into the lungs, abdomen, or central nervous system). If this is the case, the condition can rapidly become life threatening. Signs of extensive bleeding include coughing, pale gums, lethargy, vomiting/diarrhea (with or without blood), cold extremities.The good thing about this type of poisoning is that it is often treatable if it is caught early.
Another possible cause of the bruising is trauma, although if this were the case, I would expect that the dog would be sore if you touched the affected area (although this is not always the case - some dogs are very reluctant to let you know how painful they are). Bruising on the abdomen from trauma could be the result of a kick or a fall.
Given the possibility of bleeding elsewhere in her body that you may not be aware of, I would recommend that you have Butter seen by a veterinarian as soon as possible. If you wait until Monday, it could be too late to help her, or the disease may be much more serious. If there is an emergency clinic in your area, I would suggest that you take her there for an exam. If what you are seeing is truly bruising, the vet will probably want to take some blood to test for clotting problems. If not, it will be worth the peace of mind to know that she is going to be ok.