Hi, thanks for using PetCoach! Poor Bella!
Causes for scooting/dragging bottom are: full/impacted or infected anal glands, itchy bottom, stool stuck to the anus and sometimes worms. Causes of bloody and mucousy stool and a decreased appetite are many: intestinal parasites, viral or bacterial infections, dietary indiscretion, pancreatitis, toxin ingestion, food allergy or intolerance.
If she is on monthly heart worm prevention it also is a monthly dewormer making worms less likely. The only way to know for sure is to drop a stool sample off at your vet to have it tested. I can't tell if what is in her stool is worms or strings of mucous. I would start Bella on a bland diet of boiled chicken or hamburger with rice. Feed 1 cup per 20 lbs per day until 2 normal stools. Check er bottom for hanging pieces of feces. if she continues to scoot then she should have her anal glands checked. Below is a link to a good general dewormer.
I wish Bella the best!
https://www.drsfostersmith.com/product/prod_display.cfm?pcatid=1459