Jim took Katie to the vet today for a full blood panel and second stool sample test. So first they took blood from her poor itty bitty little neck.

Then Jim gave the vet a sample of her stool WITH BLOOD IN IT so they could see the blood for themselves. After that, Jim asked the vet to listen to Katie's heart again. When Dr. Jaffe did, he couldn't hear a murmur anymore! He listened to Katie's heart for about 10 minutes and then called in the other vet who works there (Dr. Kaufman) to listen to Katie's heart. So both doctors were unable to hear a murmur. They were VERY confused because they both heard a murmur Monday night and they said that heart murmurs don't just "disappear" within a few days.
Dr. Jaffe said this does not prove beyond a shadow of a doubt that Katie does not have a heart murmur. Her condition is still suspicious because she's lost weight and has blood in her stool. So we're still going to go ahead and have tests performed until we know what the root cause of all this is. So please keep my little girl in your thoughts and prayers.
As far as the blood in the stool goes, Dr. Jaffe told Jim not to feed Katie kibble or anything hard until further notice. He said we should feed her plain white rice, boiled chicken, eggs and plain yogurt. We'll know the results of the stool sample tonight around 5 or 6 p.m.
Thank you all so much for your support and advice. Please keep your fingers (and paws) crossed for my Katie.