Can't pump but can breastfeed!?!?

Ana
So for a while I thought I had a low supply but when I pump almost nothing comes out but my baby has been breastfeeding still, all day and night without complaints. 
Did this happen to anyone? Why does this happen? 
Also, I introduced her to soup and other milk (formula) as I can't take anything out, she needs other alternatives. The soup has been fine until now. She's been vomiting the other milk - has allergies to cow milk. By the advice of the doctor she will try a special milk, but now she rejecting soup, and every bottle she encounters!! 
She only wants my breasts poor thing... Has this happened to anyone? 
She's 5 months and a half, been gaining healthy weight ever since birth.