Goodness, I had these experiences 15 years ago, but it is still so vivid.
My little girl just lost weight - never had enough, and I felt like I was never finished with breast feeding. I didn't understand it at all.
In the end, I went with bottles and thickening gels, that she could keep it down, she gained weight and all was ok.
But I remember it as a traumatic time.
Hang on in there.