Talk to Pediatrician on Constipated Baby

Mother started weaning baby at 4 months (porridge, rice cerelac, banana, apple sauce) due to insufficient breastmilk and baby's refusal to take formula. Initially baby was fine but developed constipation after a week. Giving the baby skillax drops since 6 days as per doctor's recommendation but baby pooped only once on day 3. Feels like she is about to poop but she screams, arches back and holds it in. Doctor says to continue giving her drops. No other clinics nearby for second opinion. Older brother who was exclusively breast fed for 6 months due to ample milk supply also developed constipation at the age of 4 months despite every 2-3 hour feeds throughout the day. Please help what to do!