Open Question: Why did my bunny die?

i had adopted 3 baby bunnies about a month ago out of no where the smallest one died..She was just fine last night hopping around and playing and this morning when i went to change the cage feed and water them she was dead :( i took the other two out of the cage and took the dead one inside of the cage outside because i just cant bury her i still cant stop crying :( what happened she was the runt and she was about 12weeks old