My heart goes out to ya!
Same thing happened a couple months to our oldest...she had developed some sort of Tumor on her hind leg....lost tons of weight and finally after weeks of not being as social as she always was we had her diagnosed. I think she was down to 6 or 9 lbs...she was 13 or 14 though.
6 is young!
I shed quite a few tears especially when we buried her.