Breastfeeding with flat nipples

An

So before I started bf my son my nipples were flat but at about 3 months pp they “popped” out from feeding my son. My question is will they stay out or go back down to flat once I’m done breastfeeding him?

I know it’s a weird question but feeding him before they “popped” was very painful and I’m wondering if I’ll have to go through that when I have another little one.