Tonight when I changed DD's nappy she had bright green poo! It has been green before but nothing this bright!!

Any idea what it could be?? I know of lactose overload but I don't think it is that, she only feeds off one side per feed and hasn't been feeding any longer than usual.

She has been really upset the last few days, screaming more than usual. No temp, no other symptoms but crying (which she does a lot anyways)