Help me please

My little boy is 4 months old and has become really fussy lately. His weight hasn't been advancing like his doctor wanted it to and they think its because of my breast milk. I give him 2 bottles a day in addition to me breastfeeding him regularly. I have been feeling really low since his doctor has made the assumptions that my breasts are not producing enough milk. So low that I've even forgotten to eat a few times in the past few weeks. Someone please give me advice on what I should do. I went from exclusively breastfeeding to not feeling confident in myself at all.