Extreme weight loss

Catherine
Hi ladies. This is my first time being pregnant and I was curious as to how to gain healthy weight. Ive lost a total of 50 pounds and my doctor doesn't really seem to be concerned at all. I can't gain any weight no matter how much I eat and I really am concerned that with the amount of weight that I lost that I won't be able to breast feed. Is there any way to gain some weight? Any advice?