Dog not eating, loosing weight, drops of blood coming from vagina.Vet can't find anything wrong!?

My dog is a German Shepard-Pitbull mix. 9 yrs old...About two months ago we noticed she lost weight.Took her to vet, vet said she is fine, most dogs are overweight and she is perfect..A month later she kept loosing more weight, again took her to different vet this time. They checked her stool, normal. They took blood test, normal...Took another blood test to see if it is a intestine disease which is common in German Shepard's, it only showed that her white blood cells were off and vet put her on a special food to get those white cells normal..My dog keeps loosing more weight. They thought it was UTI and gave her antibiotics, not the case. She stopped eating and hardly drinks. They took X-rays, nothing showed up..Another blood test, again nothing...Finally we did and Ultrasound, they can't find absolutely anything wrong. She is home know, hardly can move, we saw few drops of blood from her vagina. Vet put her on different type of antibiotics....I think my dog is dying and don't know that to do... After $1000 spent on all the test, no one can tell me what is wrong with my dog....Please help! Did anyone experience anything like this???? I don't know what else to do.