2 13 week old rabbits, one has suddenly died what happened?

my two 13 week old rabbits have been fine since i got them. they been eatin. i havent changed there diet at all since i gt them they were growing normal, nothing out of the ordanry seemed wrong. i had them out yesterday and they were perfectly fine. when i went 2 cover them up last night i noticed one of them was lyin flat out n i thought what is she doin. i touched her and she didnt move at all her eye that i could see was blank. i checked if she was breathing and she was. i gently turned her on her other side 2 see what her other eye was like it was focusing and following my hand. i picked her up and she didnt show any sign of pain, i laid her flat and got a syringe filled with water and gently gave her a little bit of water and she drank it nd seemed 2. then 2 mins after this she started kicking her back legs then suddenly stopd moving. there is no visble signs 2 y she died. her eyes were clean and so on, she was a healthy weight so what could av caused this 2 happen?
Answers:
Only two possibilities! The first is that it died naturally as it was weak, the second being disease. You should get the other one checked by a vet and take the dead one to be checked as well. You may be tempted to just burry it but you shouldn't as you could cause an outbreak of something that could start killing cats etc.