My guinea pig died ? when i brought him?

i brought 2 male guinea pigs from pets at home they said they was about 8 weeks old but one was bigger and we brought anothor female what we was plaining on puttin in aseprat cage but on that night the smallest male died he had no bites or sractes he was just dead and we dont no why pets at home said that they ad bin cheacked and that when we got them ...... we adin bin feedin them some dadielion leafs not alot and we washed and cleaned them it had the runs abit in the day can anyone help please