Kitten biting my older cat?

Okay, so we have two kitties.
My 2 year old Domestic Medium hair, her name is Lucy.
And my roommate's 10 week old kitten, his name is Archie.
Lucy has never been violent or mean with him. She hissed at him the first few days, and ran away, but that was all.
These past several days, they've been playing pretty well.
But in the last two days, Archie has started biting her neck and making her yowl and kick at him.
He doesn't try to mount her or anything, he just bites her.
Lucy will try to groom him, and he'll bite her on the neck.
Any idea why this is happening?
It may also be worth mentioning that Archie has adored her since day one. He's followed her around, and taken to sleeping in my room instead of my roommate's, because Lucy is there.

 

You have to spent some time with them and try to play both of them together by which they also try to play together and he forget it to hissing and groaling him.