Girls, maybe a stupid question... I've never expressed milk before. Th…

Girls, maybe a stupid question... I've never expressed milk before. There was only one case when lactostasis started, but I expressed it in my chest until relief. Today I talked to a pediatrician, and I described the whole situation to her, that the baby is constantly breastfeeding, eats two breasts at once sometimes, and actually sucks out the first completely and cries, I press, the milk doesn't come out, so I give the second breast. The pediatrician today advised me to express milk to see how much of it comes out and then feed the child with expressed milk... In general, I managed to express only 20 ml in 15 minutes, more doesn't come out. Is this normal? I just assume