So things are not as dire as we thought. I underestimated her weight she is actually about 7 pounds right now, so about 2-3 pounds underweight. They ran a full blood chemistry checking her thyroid, and for CRF, they also ran a urinalysis. All clear. The vet did, however, find some dental issues, mostly gingivitis, severe enough to cause pain while eating, which is why the sudden drop in weight. After a shot for inflammation and some antibiotics she is already eating more and seems to be feeling better. Hopefully this will be the answer we are looking for.
The reason for the weight underestimate has to be the fact that I have a hefty 12, pound 10 month old, solid as a rock, kitty (otherwise known as Rowdy), Smokey (sisters senior kitty) seems sooooo small compared to him. The vet said that he will be a big boy, possibly around 20 pounds! He is super lean and muscular... but this is another post entirely!