Is this normal?

Alright ladies, I need some advice or opinions. I'm currently 4 weeks with baby #2. We just found out on Wednesday. Ever since last week I've been SO tired. all I want to do is sleep. And my back hurts so much. Also, I am very nauseous. I don't remember symptoms coming on this fast with my son who is now two. And I never had morning sickness with him. I was up for a good most of the night last night because I was so nauseous. Is this normal? It's making me think it could be twins. Or am I just crazy? Any help is much appreciated!