Laying outside the box

I have three chickens, and one has laid regularly since October.  Last weekend another one laid Sunday and Monday and then nothing since I thought.  However, something caught my eye this morning when I was letting them out and I found a nest under a bush with seven eggs in it!  I removed the eggs and the Bluebell immediately jumped into the place.  When she hadn't moved after a few minutes I thought perhaps she was trying to lay there and removed her to a nest box and shut her in.  This just distressed the other chickens who marched up and down outside the box squawking like a picket line!  I've let her back out and put a plant pot where the nest was to try and discourage her.  

Why would she have laid under a bush instead of in a nice warm house at this time of year?  Is she going broody?  We have no cockerel so no chance of babies.  Are eggs safe to eat if they have been exposed to the elements for the past week?  Hope you have some ideas cos I'm confused  :?