She is small in general but healthy weight for her size at her 4 month visit. 24in and 13lbs. Due to low supply and some medical issues I started supplementing with formula, which she took to surprisingly well. She has always been a cluster feeding and only eats small amounts more frequently. However, now regardless if it is formula or expressed milk she will usually only take 2oz at a time, sometimes 4oz, and feeds about every 1.5 hours. I try and offer it more frequently and she refuses! Lately she only eats about 12oz during the day and nurses 2-3 times at night for about 7min each. We introduced solids but she doesn't take much, maybe 3 spoonfuls. The doctor didnt seem concerned about her weight, but I am really worried that she is taking half of what most babies take!!! Any advice?