Baby not feeding well 😔

Pa

Hi moms...my baby has been feeding for only 5 mins after every 1 and a half hour. Due to this her weight increased by only 100gm (3.5 oz) in the last 2 weeks. The ped says that we'll have to start giving her formula since she is not drinking as much as she should. Now I'm trying to express after every feeding session and give her that as well. Is anyone else also facing this problem? I don't want to start giving her formula. Please advise.