Why do I want another baby already??

My son is 4.5 months and hes amazing. My husband says were done. We both have daughters (13 and 7) from previous relationships. I keep having this urge to have another and soon. I want my son to have a brother or sister to grow up with. The kids are all so far apart in age. I thought after I got my boy Id be happy to be done. In some ways I am, but I cant help feeling like this. Im 31 so still young. Incidentally, my husband doesnt want to use birth control except for the pull out method. He is very good at it and weve used it for years. When we ttc I got pregnant second try. Weve never had a scare though. My husband insists no more. Today w had sex and Ive been using OPKs and tracking my cycle. Im ovulating and he pulled out but doesnt when Im not ovulating. He asked me if I was and I said yeah so pull out. Its weird though knowing I have the power to say yes or no. I would never trick him ever! But its like whoa...we could get pregnant if I wasnt right. Im confident we are fine. I just wish this baby fever again would go away!