What is wrong with my poor babygirl dixie ?

I got my full blood boxer pup a day befor she turned 6wks she was alrdy wingnd off mamas milk cause the mama dog wouldnt let the pups nurse no more cause they would make her tatas bleed badly cause of their teeth. So when i got my pup she was eating wet food and dry.. And we let her sleep with our female dog we have she took in the boxer pup as her own baby. Well we think we made a mistake cause the pup when we got her would whine at the door if she needed to go potty but since we wont let her sleep with our older dog now (the pup would bite on her neck so much it bled) she did okay the first few days we put a teddy.bear n a bed in there for her n she strtd to poop in her cage then roll around in it n eat it :( whyy.. Haha yes i know she only a pup but she has been doing so good and knew to go outside so y now? I know she will have accidents but we could take her out n she go pee n poo and we would wait a bit longer and she would come in and poop on the floor. We always make sure we take her out alot we even have a doggie door she knows about but the past wk she been stubborn and pooping in her cage to roll around in it is this normal?? Accidents do happen but