Why has my guinea pig started biting me?

DO NOT listen to Justin Bieber there... dumb.. anyway I run a Guinea Pig Rescue so I have some idea of them lol... being sarcastic. Your guinea is testing you. He probably doesnt like the nail clipping and is letting you know. As long as its a nip and not a full blown bite followed by teeth clicking then then he is testing your taste. Were you hot at the time? you skin may have been salty from sweat, the smell is good and he was checking it out. It has nothing to do with adolescence or hormones. All piggies taste occasionally. I would say NO loudly and put him back and try again later when he is calmer. Its not a male or female thing either. He might be feeling stressed and scared. Nest time try sitting on the floor when clipping nails, and petting him he might be afraid of being held so high up. Wrap in a towel also this helps stop the struggling and makes them feel much safer. If he is out right biting and drawing blood then i would say you have a temperamental pig and that can sometimes be hereditary cause most people like JUSTIN up there let any ole pig breed and not care what there temperaments are.