What Causes Green Coloured Stools During The Third Trimester Of Pregnancy?

Hey, I know it's gross.. but I am currently 35 weeks and 3 days pregnant, and have just gone to the toilet with a belly ache and have pooed out a dark greenish poo.. was just wondering if I should be concerned about it as I haven't had it before? is it a sign of labour, or anything? just curious to what it is, whether I should be concerned and why it's happened.. thanks :)