Could she be depressed?

i had two rabbits and one died yesterday we have no idea what happened to the other rabbit but we found her in the cage and she was really floppy and had shallow breaths she was pooping and peeing normally and eating only hours before i checked her after she died and she had no broken bones and way i cleaned out the cage incase it was a spider bite and checked out the other rabbit who seems fine !she is also still eating and drinking.... but i was just wondering as they were both 12 weeks old and brought from the same breeder (from different litters) but have been together for a while would the bunny miss her mate ? she has been aggressive towards my mum and brother (biting and scratching) and will only allow me to pick her up and hug her and she snuggles under my jumper and stays there, she is still eating and drinking i stay and watch her eat and drink. she still hops around and to make up for the other bunny not being in the cage at night i have put a box with a warm blanket in which she sleeps in just so it keeps her warm and she has something to snuggle with! i just don't want her to get depressed and die .. it was sad enough to loose my other bunny ! i don't want to lose her to !