frustrated. baby only nursing for 5-8 minutes at a time

Kirsty
My 12 day old baby had. Even such a great eater, until today. She'll only nurse for 5-8 minutes at a time. I don't understand why, and it's so frustrating. She's doing fine with her weight gain, but I want to make sure she's eating enough and that my supply doesn't drop. Has anyone had this happen before?