An ouch or ah-ah, especially if said in a loud or high pitched tone, can make some puppies more excitable, almost as if they see it as an invitation to play harder. Watch her reaction when you do it and if you think it's making things worse try diverting her attention on to a toy instead.
Puppies tend to bite more when they're tired or overstimulated. At the moment Lucy should be sleeping as much as 20 hours a day. If she isn't getting that much sleep, and especially if she's also overstimulated either by playing too much with people or Chester, or simply by being in a busy household where there's always something going on, she will lose the very small amount of self control that she has and will become a gremlin! Think overtired and overexcited toddler and you'll get the picture.
Add teething into the equation, and the fact that puppies don't have hands so have to explore with their mouths, and you'll see why it can be such a difficult time for owners. If you can give Lucy periods of calm and make sure she has plenty of sleep that should help a lot.