5 months pp very petite and lost 20 pounds HELP

I've always been a very petite person and only gained 9 pounds my entire pregnancy but lately I have been loosing a lot of weight and I can't really afford to loose weight. I went to the doctor the other day and I weighed 88 pounds (disgusting I know) I have always been very self conscious and now hate looking in the mirror. My daughter is gaining weight just fine and I eat constantly!! I even supplement with boost. I'm really not sure what to do as of this point I feel like my daughter is taking all my nutrition from me. My doctor said with breastfeeding and being so active at work I'm burning a lot of calories and I'd just have to wait it out until I'm done breastfeeding to gain it back. But I feel like I look disgusting and can't stand it i really really want to breast feed until my daughter is a year old (she is 5 months now) but I also have to think of my health. Any suggestions on what to do? Or how to gain weight?