Hello Doctor My Girl Baby Is 6 Month 20days Old...we

Hello doctor my girl baby is 6 month 20days old...we hav introduced forumal milk when she completed 5 months...we are giving 90ml of forumal for daily once...on other times she is completely breastfeeding....didn't introduced solid foods yet...but my baby is not going stools properly. ..she is going stools every four days once only from last 3 weeks...is she have any problm in her stomach? how to cure her?? We are very confused and worried..she's drinking plenty of water on regular intervals...kindly please suggest me wat is d actual problem and how to solve this???