i need a little encouragement

things have been going really well with pepper my ggc but today she has been really stroppy and done nothing but bite (she reeaaally hurt my arm!) the "little earthquake" technique had been working well but didn't do anything except pee her off today at which she bit even more. i've been keeping her off my shoulder and last night she had even half mastered a little wave so i have been getting increasingly disheartened today and feeling like its all going wrong.
the only thing i did differently was buy her a little cozzy bed thing that she loves and i think she has decided that it is snugglier than i am and so doesn't much want to entertain me at the moment. i know this is going to be an uphill battle and a long process that requires patience and i know its only been a week since i got her but it doesn't make it any less disheartening.
also the pet store said she was only about 15 weeks old is there any way i can check this? she does have lots of fluffy grey feathers that you can see on her chest and under her wings and also a lot of new feathers coming through, does this help in guessing her age?