I can't say for a fact, but my 5 month old Cavalier King Charles had blood and mucuous in his stools regularly from the day that I brought him home. My vet said he has collitis, and have been feeding him chicken and rice (no dog food), keeping treats out of his diet (except for cheese and wheat and gluten free treats) and that has settled him. I'd take yours to a vet if it doesn't settle soon. Good luck!
My little girl had the very same thing and after tests it turned out she had something called camphilabacta (spelling). She had a course of antibiotics which helped at first then it came back a couple of months later, she was put on a stronger antibiotic and has now thankfully cleared up. I am lead to believe that it can be a result of the water being too hard or soft I'm not sure which, however as we bought our little girl about 200 miles from our home the risk was probably less to her after we got her home then when she was with her mother. I spoke to the breeder about it and she said they had a problem with it there and asked my advise on how we cleared it up.