Why I am so emotional?

I'm six weeks pregnant with my first and everything is so new. I don't mind going through all of this pregnancy stuff, I just hate how emotional I get everyday. I'll be completely fine one day and then next I'll wake up crying for no reason. For example, this past weekend I was completely fine, strong and could hold my emotions but this morning as my fiancé got ready for work and I sat in bed bawling like a big baby because I missed him already. He hadn't even left yet. I feel weak when I'm emotional and I don't want to be emotional. I have been crying off and on since he left. He called me while he was driving to work and I cried the entire time we were on the phone. I feel pathetic. So I have deemed today an emotional day for crying.