I got 2 guinea pigs in january and today 1 died suddenly and i dont know why?

they were both 8 weeks old when i got them. scruffy and smokey. the day that i got them i noticed scabs all over scruffy and took him 2 the vet and found out he had been attacked in the pet shop by the other guinea pigs he was given sone antibiotics and was fine after and smokey was fine. yesterday they were both fine and happy but this morning when i opened the cage i found smokey dead he was cold and stiff so he had been gone a while. i give them fresh veg and dried food, hay and fresh water everyday so i dont understand why he died does anyone have any idea what it could be because i am very upset and really need to know what caused this. and does anyone know what should i do with scruffy because he has started biting which he hasnt done in a fue weeks and seems very withdrawn. any advice would be really helpful.