Unfortunately, I have experienced this with Bailey. It is the scariest thing to see your dog poop blood in large amounts.
It happened to Bailey at the end of June. She started off with diarrhea, she refused to eat, threw up and then become lethargic. I took her to an emergency vet. She was diagnosed with gas, given a shot for the vomiting, metrondozle for diarrhea and was not allowed to eat or drink for 8 hours. She was starting to feel better and then a few hours later she started pooping blood...pure blood. I was terrified. I called back the vet. He said this was normal. (Didn't seem normal to me!) He said no food for 24 hours. She continued to poop blood throughout the night, but it got less and less. I stayed up with her the whole night. I took her to my normal vet the next morning. She was all ready starting to feel better by then. It took a while for her bowel movements to go back to normal. About 2 weeks later, she pooped blood again. We got more metrondozle from the vet. It finally went away.
I believe she picked up a virus from the park. 6 days after Bailey had bloody diarrhea, my parents' maltese started pooping blood and their chihuahua started having loose stool. We took them both to the vet and got them on metrondozle. Thankfully, they recovered a lot faster than Bailey did.
I advise you to not let Meggie and Cooper share a water bowl for the next few days. If she did pick up a virus, you don't want her to pass it on to Cooper.
I hope Meggie feels better soon!