How old is your girl?
When did this start?
Has she had any bones, raw hides or non food items lately?
Any other symptoms? (Vomiting, lethargic, gas, or diarrhea)
Constipation could be the cause of the blood but mucus is not a symptom of constipation so it is likely something else causing the problem. Either way unless this was a one time deal and a small amount of blood she will need to be seen. If it was only once and it was only a small amount then you can feed the bland diet and place a couple teaspoons of canned pumpkin (Not pumpkin pie filler) into her bland diet and monitor her. Should you see more blood, or if this was more then once, or a good amount of blood then you really have to have her evaluated by her vet and a fecal smear done.