Talk to Pediatrician on Baby Weight

My daughter is 2 months old she is 2.6 kg her birth weight was 2.3 kg .she is not gaining weight dr suggested reducing substance test for stool which came positive.she only takes 10 ml milk 2 hourly previously she was on breastfeed but now formula after taking feed she becomes extremely irritable always crying passing lots of gas but stool only one time a day . please help what to do how she will gain weight