Puppy having diarrhea after switching from his puppy food

[DeAnah]

My 11yr old Yorkie has recently been leaking a bloody, mucusy stool...Looks like her bowels are a bit inflamed...She was recently taken to the vet for a teeth cleaning, blood work, shot update, and X-rays.. Come to find out she has COPD, water in her lungs, and an enlarged heart. She was put on a low sodium prescribed canned dog food called H/P for cardio health, Furoquid for fluid retention, her other meds were for pain and antibiotic due to 5 teeth being extracted...these procedures were done over a month ago. I have recently taken her off Furoquid and added soft "Blue" (which I always fed her Blue Senior Kibble) dog food to her new prescription dog food due to her not eating it, just to give it a little flavor. Because of the recent leakage I have strickly given her the H/P because I was too worried and thought it would be the healthiest for her until I can take her to the vet...I'm at a loss! I did also just pick up the Bene-bac (that most recommended on here)and sprinkled that on her food day before yesterday, because she was dripping blood after a stool. I will be taking her in this week, just wondering if anyone has had similar issues with their baby? Thanks for your time!!