I found blood in my guinea pigs' cage. What could it be from?

The guinea pigs could have had a fight and one could have got cut or scratched. You should check out both guinea pigs and see if there are any marks on them that could have drawn blood. You could also watch them togther in their pen to see if they actually do fight. If they do then you should probably separate them into different pens. I don't want to worry you, but if you don't find a scratch then there could be a medical problem with one of the guinea pigs so you should contact your vet.