Can someone please tell me or give me some idea why my dog should have green poo...(sorry it isn't a very nice subject)

This can be a sign of infection.   I think you should consult your vet taking a sample with you.

How long has it been green? What do you feed her on? The reason I ask is cos one of my dogs' was like this for months when she was on a poor diet. Not that I'm saying your dogs diet is poor, like Isabel said it could very well be an infection, just curious thats all.

Dee

Have seen dogs pass green stools after eating coloured chew sticks, have also seen green stools after dog has eating rat poison-- go to your vets