Cause of gerbil death?

my gerbil, layla, very unfortunetly died sometime last night. i woke up early this morning, her vet appointment was this morning, to wake her up. she was stiff and cold. i told my mom and we buried her outside. she was very well loved and adventurous. but i really want to know what was wrong with her. it started off very gradual, she never realy ate much, unlike my other gerbil, sandy. my mom said that could be part of the problem. and last night, i was holding her and she was cold, and desperate for the warmth of my hands. (like id put my hand in and shed hurridly crawl in) she didn't run in her wheel as much when her nose and eyes started getting crusty red stuff around them. she hunched over, and even ran with a hunch? she started eating even less and drinking more water, and sleep a LOT more. last night, i wrapped hher in paper towels, but i could tell she wasn't going to make it because sandy wouldn't sleep with her. but i still prayed she would live. another thing, she didn't cough, sneeze, and didn't have a crackling noise when i held her up to my ear. she also tried to run in her wheel but would trip and i noticed her right side was a little limp. she would also fall over when she tried to drink water and i had to help her. pelase tell me what was wrong with my little layla. thank you