This can't be normal right?!

This isn't 'normal' right??
I am finding myself increasingly anxious each night. I lay for hours scared to fall asleep incase something happens in my house. From break and entering, stealing my car all the way to my children passing away in their sleep or even in a car accident the following day. Driving is becoming a nightmare as I am so stressed and anxious something is going to happen to my poor kids! I have a 12 week old, 3 and 5 year old.
I have to check my children multiple times each night and also check the locks on my house each time. I don't live in a particular area with a high crime rate but still for some reason it controls me every night! I'm a single mother and I am exhausted! I am going back to work in a few weeks and I need to get this sorted, I can barely function as it is... Is this just apart of being a mum? Ways I can help combat this? It's only really gotten really bad since my youngest was born, is it possible this is a form of post natal depression?
Help please!!