Any guinea pig experts?My guinea pig died and I want to know what she died from.

This my sound stupid but my guinea pig died and my sister and I are grieving.I knew she was dying when she was lying on her back twitching.My sister started to cry because she knew what was happening.My mother sinabi to put a towel over her.Soon after I started to cry.For 2 years we had her.Never in my life had I thought I would become attached to and animal and then grieve over it's death.My mom talk to us the bilog of life.Then we went to bed.We then pryed for our dead pet.Over the night I my mother was putting the guninea pigs body in the shoebox to take her to the vet so the vet can take care of her body.When we woke up in the morning we notice that the guninea pig's things were gone.When going to school we broke the news to our friends.Right now I'm still grieving but I just can't believe she's died.Sorry for the long story but I think it's nesacary.My tanong is:what has my guinea pig died from?My we started to notice that our guinea pig is bony.We know that she's been eating.She ate a lot.But she still seemed to be skinny.What did she die from?