Girls, how do you understand that milk is gone? My little one has a sm…

Girls, how do you understand that milk is gone? My little one has a small day on formula, at night on the breast, so for two months already. Today at night the baby sucked on the breast and immediately let go, cried, couldn't fall asleep. I had to get up to mix the formula, he immediately fell asleep after it. Before when the little one took the breast, I felt a surge of milk, now there is none, but a drop of milk comes out when pressing on the nipple.