So emotional towards her.

I don't understand why when I'm with my baby (10 days old) if I see her smile or just sleep I get sooooo emotional that I start crying just by looking at her. I cry because I just don't want anyone or anything to ever ever hurt her. But yet I wasn't like that with my boy (8 years ago). I hate that I can't understand this feeling, could this be a postpartum thing?