Why do I feel a really urgent need to be pregnant?

I dont understand why, but I constantly daydream about picking out baby clothes, going through pregnancy, how I would tell my family and friends I was pregnant, etc. Ive had dreams about being in labor, helping a friend through labor and so on. I have no idea why, but I feel a pang inside when I see a newborn. Im only 21, and I havent finished school just yet. I know there is no way that I am ready, but its like a subconscious thing! I have had a boyfriend for over 4 years now, but I havent told him I feel this way all the time. Why could this be?