I'm 5'6'' and 100 lbs. I eat all types of food, but can't seem to gain an ounce.!

I'm 5'6'' and 100 lbs. I eat all types of food, but can't seem to gain an ounce.?

I gave birth to my daughter in November and have been breastfeeding her ever since. Breastfeeding does require extra calories, so I've been trying to eat more snacks in between meals. However, it's difficult to find time to snack with a 7 month old.
I've always been slim, but never boney like I am now.
I have to carry my daughter everywhere we go because she insists on not being put down for a moment. She's nearly 21 lbs right now, so imagine carrying a 20 lb. weight with you everywhere you go.
My question is, though, what types of food can I eat in attempts to gain a little gain? I don't want to gorge on cookies and ice cream.. I want to gain it in a healthy way. It seems like no matter how much I eat, it doesn't change anything.

protien shakes wont do it. If you wanna gain weight fast, you need to start eating very high carbs.
from a biochem stand point, you have 3 items ur body utilizes. carbohydrates, amino acids (protien), and fats. Of the 3, fats are ok in moderate amounts because you need them for the synthesis of hormones and stuff. However, the body has no limit to how much it takes in for the fats, and the prob is, once it gets into your blood, your tissues only take up oh so much of it, and ur liver can only reuptake so much lipids. hence - ur arteries get clogged and bam heart attack. amino acids, all thats gonna happen for the most part is you will break them down and have a positive nitrogen balance in your body. yea some will be converted to fat, but not as much as u think. carbs, now the story with them is that they are the main source of fuel in the body. however, when you have extra, your adipocytes (*Fat cells*), in the presence of insulin, will convert them into DHAP (* dihydroxyacetone phosphate*) and then to Glyceraldehyde-3-Phosphate, and then into triacylgylcerol. Now yea, its fat, but its not in your arteries. So hence you will gain weight.
Bottom line - Eat more carbs.
Reading a biochemistry book isnt bad either but it looks like ur strapped for time.

-The Med Student