Feeling... Off?

Well.. I got my BFP on Thursday July 9th. My due date is March 10th. Since then I don't feel like I am truly happy about it. I know that sounds so terrible and ungrateful, but I am so scared and nervous. I have an almost four year old son, and I am 25, so it's not as though I am inexperienced. I don't know why I'm feeling this way, but I can't shake it. I feel like I couldn't love another child more than I love my son.. I'm having such silly inhibitions, but it's consuming my mind. I always thought/knew I wanted a second child, I just am not feeling how I thought I would about this pregnancy.