TTC #2

My daughter is 7 months old and my husband and I have always wanted a close in age family so we started trying again when our daughter was about 2 months. I had the impression that when you have had a baby, you are super fertile and can get pregnant really quickly again? Im not breastfeeding and i cant see an explanation as to why we arent getting pregnant again? I read something about second baby infertility syndrome.. Was wondering if anyone had any tips or even words of encouragment as i am starting to worry ?