Hello everyone. The baby is almost 2 months old and I'm breastfeeding …

Hello everyone. The baby is almost 2 months old and I'm breastfeeding her only. Two days ago, I started having very little milk (usually a lot, I even pumped it). So, two days ago there was very little milk, the baby didn't eat well and didn't sleep. According to the advice of a doctor, we bought Similac Classic. On the first day, I gave it only before bedtime so she would sleep well. On the second day, in the afternoon. And so on, all the time at the breast. And now for a day, she hasn't gone to the toilet, constipation. What can be used for constipation in 2-month-old babies?