Please help my cat. She is sick.?

Ok so my cat is afraid to come down stairs because she thinks the dogs with chase her. She had started pooping blood in my bed (this is soooooo not like her), so we took her to the vet. The vet told us she was fine, she was just stressed. So we put a litter box in my room so she never has to go down stairs to use the box. She just seems to be getting worse. The poop has become extremely runny, lots of blood, and it smells SOOOOO BAD. When she gets into the litter box, it sounds so weird when she goes poop, like she is farting. Im so worried I have had this cat sense I was in kindergarten. She is my best friend. We are going to take her to the vet on monday. But in the mean time what do you think is wrong with her, and what can we do?