Hi Emma, hmmm what are you doing at the time Stella does the Nibbling ??
Misty used to do that if I was stroking him, and HE decided He'd had enough, it would start out
with a gentle swipe, then maybe a gentle nibble till I stopped
But Boy, If I didn't stop
( You could see this Mad / wild expression come over his face )
It was then that He would get really rough and Lash out
so if it is that, I can only say to always Remember to stop when Stella tell's you, Don't forget..... " SHE is Your Master "