grumpy guinea pig

[Laura M.]

Recently, one of my guinea pigs has been really grumpy. He was really friendly, but now he bites me, and he bit Neville, his bonded brother, on the nose,leaving a bald patch!
Is there a particular guinea pig cause?

 

[Lady Kelly]

Biting can be a pain response so as he is exhibiting a change of behaviour I would take him to a vet for examination