unexplained crying

Today I found myself inexplicably sitting on the floor in the living room crying. The sitting on the floor was explicable- I was playing with Jonathan on his play mat. The crying was the weird part. Jonathan was happily giggling and cooing and being wonderful and adorable- sucking on his feet and rolling around. I was alternately making faces and singing to him and looking up a question in one of our baby books. Only instead of grabbing the baby book off of the shelf, I had grabbed one of the pregnancy books. As I was paging through and got to the section on "What to bring to the hospital", I burst into tears. A lot of tears and a sob or two. Why? I don't know. I just found myself sitting there crying. And then I was done and everything seemed back to normal. Maybe it is the lack of sleep. Maybe it is that I am done with the pregnancy thing (for now at least) and feeling nostalgic. Maybe it is hormones. Who knows.