why do I want another baby so soon!?

My baby boy is almost 3 months. I hated being pregnant at the time but I had a fairly easy pregnancy and birth. As I say he's only 3 months now and I feel like I would love to get pregnant again already.. I'm too tired and still not in the swing of having sex properly again so it's hardly going to happen.. but why am I feeling this way? Every time someone else announces they are pregnant I feel so sad. I couldn't wait to NOT be pregnant, and now I am I want another already? My OH says he doesn't know if he ever wants another one yet! I didn't feel this want to have a baby when we were first trying!
Is this a hormonal thing? Has it happened to anyone else?