Can I make myself “narrower”?

I’m 4 months postpartum and 5 pounds less than my pre-pregnancy weight (yay!). I didn’t have a very defined waist before pregnancy, but now I feel like I have nearly no shape to my waist, like I’m just straight up and down.

Since I’m breastfeeding, I’m not looking to do much overall weight loss, I don’t want to be in a calorie deficit. Are there some exercises that would define my waist more, or is my waist permanently widened?