Breastfeeding while pregnant, worried about eating enough

abbyful

We plan to try for baby #2 when DS is somewhere around 9-12 months old. I have some concerns about my future pregnancy if I'm still breastfeeding him.

I know they say it's perfectly fine to be pregnant and breastfeeding. My concerns are that I'm already technically underweight, and have gotten to even be a few pounds under my pre-pregnancy weight. (My sister had this too, she lost so much weight her doctor told her not to breastfeed anymore.) I didn't gain much weight while I was pregnant either, 18 pounds total I believe, DS was 7 pounds 15 oz at birth.

I eat A LOT. During my pregnancy, I didn't gain any weight my 1st trimester even though I was eating 1-2 additional full-size meals a day. And now that I'm breastfeeding and my weight has dropped a couple pounds below my pre-pregnancy weight, I'm consciously trying to eat as many calories as I can.

For example, here's what I've eaten today so far, plus I'll have a big dinner:
Breakfast: Carnation Instant Breakfast shake made with whole milk, 1 spoonful coconut oil
Lunch: half a T-bone steak, 2 cups mixed veggies with 1/4 stick of butter, 4 oz of applesauce, ~3 tablespoons peanut butter.
Snacks in the afternoon: some chips & queso, slice of summer sausage, piece of cheesecake.

I eat all this food just to try to not lose any more weight! 

Will this be a problem if I'm still breastfeeding when I get pregnant again? Will I be able to properly nourish a developing baby while still breastfeeding DS?