BIG HUGS
I am ever so sorry. A post mortem examination is a good idea if you have more guinea pigs to make sure that it is not infectious or if you are desperate for answers that none of us can give you.
Guinea pigs are prey animals; that means that they will suppress symptoms for as long as possible and to a rather amazing degree. Being more subdued and drinking more or drinking less - usually corresponding with a weight loss when you weigh daily - are often the only warning you'll get that something is not right and needs to be seen urgently; especially if you have just experienced a death which is additionally lowering the immune system. I always switch to weighing newly bereaved companions daily and check on them more closely, especially if the companion died unexpectedly and without a clear pointer as to what had actually gone wrong. It is always an anxious time and hugely upsetting if you lose two or more piggies in quick succession.