Why does this hurt?

I was just on the phone with a good friend of mine, and I think she just let it slip that her sister is pregnant. It felt like someone kicked me in the stomach. But the weird thing is that I am not even friends with her sister. I do not begrudge her happiness or children of her own. Why does the thought of someone else that I *sort of* know being pregnant bother me so much? It's not like I think the conception of all new babies must cease now that I am no longer pregnant. Blah, this sucks. Silly me, I thought I was having a good day.