Hello and welcome!

I would take her to another doctor and get a second opinion.

sounds like it could be reflux, although there's prob many other reasons why as well, such as teething which although is rare to happen so early on, it can.

have you visited the doctor? what was the outcome?

hello I am a mother of twins and that was how mine wwere 1 good eater and one not so good. You say you have tried other formulas have you tried other bottles or nipples for that fussy one At one time my girls were on different formulas and different bottles and eventually after six or seven months they finally used the same now until this day the fussy one is still fussy will not eat somedays and our doctor says a child will never starve itself. try not to worry if then fussy twin is loosing weight then take her to the doctor soemhting maybe wrong if she is gaining nothing to worry about some babies eat less than others. You can PM me if you like I went through this already. I have a guestion where do you live?
Taykare