baby not gaining weight HELP!!!

Monica

I stopped breastfeeding 4 weeks ago.. I'm feedimg my 3 month old Similac advance.. I took her to the doctor and she stopped gaining weight.she lost a pound. she barely eats 3 oz at the most. they just changed her to another type of milk but will be doing a test to see if the milk is going down the right way. has anyone gone through this? I feel so bad and guilty