Newborn constantly pooping & asking for more feeds

Any newborn mummies experienced feeding breast milk and baby is constantly pooping then asking for more feeds? I’m feeding breast milk and formula concurrently because I feel that my breast milk is causing baby to have diarrhoea(???) She’s only 21 days old but is on 90 ml of formula and hungry for milk again in less than 2 hrs!!! Is it normal and will she be over fed?