I think hormones may definitely have something to do with it. You already know how they can make you crazy. You can't make them stop, but you can be aware that they're after you.
Try to get your mind going in a different direction. What can you be happy about, thankful for? That your husband has a JOB! That's a good one. That you have your son to take care of, and a new little one in a few months. That you are safe from the winter weather.
Get busy! Rest when you must, of course, but plan your days very well. Get your housework done. Play with your little boy. Is he old enough to draw some pictures to surprise Daddy with when he returns? Have him do one every day. Take him to the library. Bake some cookies, if it doesn't, um, disagree with you. Do you have a nursery to plan? Start doing that. You'll probably change your mind several times before #2 arrives, but the planning can help your emotions keep steady and maybe keep the tears away.
I know it sounds corny, but attitude and activity can beat the blahs, and sometimes they can even beat the hormones, too.