Poor feeding and poor weight gain. I do?

My daughter 4 months old (bw-2.75) had poor feeding since birth. She never sucked breastmilk continues for more than 5mins & she drinks only from one breast. She used to sleep off when she started feeding. Tried giving her lactogen/nanpro/lactodex/expressed breastmilk as bottle feeding, but she refuses all. Now my milk supply has gone down completely. She cries with hunger, sucks hand n rooting reflux present, but no feeding. Till 45days of age drinks 80-90 ml once a day but now Drinks maximum 40-50 ml if we forcefeed her. Occasionally she drinks some milk during sleep. M/H- fever at 3 days of birth(blood culture normal, taxim and amikacin inj, urine culture was not done) Fever again at 45 days of birth(blood culture normal, urine culture positive for staphylococcus and ecoli. Amikacin and oflox inj ) Milestone - rolled over. Weight- may 4- 2.75(BW) June 16- 3.650 June 25- 3.87 July 17- 4.45 July 27- 4.66 Aug 18- 5.00 Sept 9- 5.20 Why is it so? What can I do? I'm totally scared.