bittersweet emotions

I have an eight year old son from a previous marriage and am now happily remarried and expecting a new baby girl in 6 weeks. I have been having some bittersweet emotions thinking about the time I've had with my son over the past 8 years just him and I and now him no longer being my only "baby". I started crying today thinking about it. I am very excited about her being born so I am not sure where these are coming from. Has anyone else had a similar experience?