Pregnant after twins.?

I have just found out I am 4 weeks pregnant. We have been trying so it wasn't a surprise. The surprise is how I am feeling. I thought I would be excited straight away but I am petrified!! I have 4 year old twin girls who are due to start school in September and I am just so worried at how this will affect them. I want to give them the very best in life and am worried that they are going to have to go without some things because we have a baby in the house. We have spoken to them about a baby and they are fine about it so why am I feeling so worried??? Is it normal to feel this way?