baby poop. . .

My 2 mo old has always been supplement fed, I pump half and give half similac supplement which bad prebiotics to make their poo sinilar to breastfed babies and her poo has always been watery like itssupposed  to be and went after every feeding and now as of like 2 or 3 days ago she has only been going like once and its alot more thick!? Its making me nervous that she hasnt went number 2 yet today... Should I worry? She doesnt seem uncomfortable..