Son only wants his Dad

Slowly over the past few months my son has grown to want his Dad more and more. If I pick him up he cries and reaches for his Dad. He follows him around every where and won’t interact with me unless his Dad is gone. When we are alone we have a great time playing and having fun, but as soon as Dad walks in the room Mom is thrown aside. It seems to be getting worse and not better, so either this isn’t a phase or it’s a really long one! It’s so hurtful to be pushed away all the time, the past few days I just sit back and watch when his Dad is around, and try to get my time in when he’s not, like during the bed time routine that I have always handled. I am also three months pregnant and overly emotional, so that doesn’t help. Any one else gone through this and have advice?