My daughter had a little chihuahua pup that we rescued and she had an autoimmune disease.  We had a rough first year, but then it seemed things were good.
So, the pup is healthy and  situated now?
Well, we made it 6 years and then she got pancreatitis.  She recovered from that but then we noticed her being really sleepy.  Turns out her body had started destroying her red blood cells, we tried for a few weeks to help her, but ultimately there was nothing more we could do.  I still think about her daily.
I'm sorry to hear that RIP doggo.   I have a rescue that's middle aged now. 