TTC while breastfeeding?

We want our children close in age, our baby is 1 year old and we've been not so much trying but not preventing either since we had her. I'm still not pregnant and I'm starting to get concerned as I fell pregnant fairly quickly the last 4 times (we had a few miscarriages before our daughter). I'm still breastfeeding on demand but I thought the only reason breastfeeding could delay pregnancy was if it delayed periods and my period came back regularly 1 month after birth. Has anyone experienced this or found it harder to get pregnant with their second?  😔