
Ideally she needs to be sleeping in her whelping box now so that's she's happy there when the time comes. Can you not make a secure area for her, larger than a cage, so that she has more freedom of movement?
I'm not sure what you mean by her behaviour becoming aggressive. Bitches become protective of their litters but not aggressive - unless they had poor temperaments to begin with, in which case you wouldn't breed from them in the first place.