B.,
It sounds to me like a couple of things are going on, none of which are your fault or abnormal. One of the issues is that he is choosing daddy over you because kids do that. They choose different times in which each parent is the preferred parent. It also can be due to the fact that he has you more often than dad so he wants to get his time in so to speak. Another factor could be jealousy of the new baby that you are carrying. Kids are really smart and I am sure you and your husband have been talking about the baby and some of the things that are going to happen when the baby comes and so he is feeling that the world is being turned a bit and mommy is carrying the reason. He will adjust and you again will be his chosen parent before you know it. If he is anything like our son he will choose when it is most difficult (wanting to be held while you are feeding the baby).
Good luck, congratulations. This too shall pass and don't doubt that you are still the one he will want when he isn't feeling well.