Am I missing an obvious reason for a collie's appetite to go bananas?
She is 5, can be a bit greedy and often nibbles on grass/licks soil

As of last Thursday-ish, she's been ravenously hungry, nearly taking my fingers off when I offer food, and inhaling her main meal in seconds.
What is niggling is that she's not put on weight - if anything her ribs are more prominent. I'm going to dig out the scales if I still have them and weigh her tonight.
I have wormed her (Drontal) and if she's like this during the week then off we go to vet. I'm giving her extra food!
No significant change in routine, exercise, weather...?
Thoughts?