I have two guinea pigs. One has diarrhea, and the other one is cleaning her butt. Bring your pig to the vet at once for treatment. Keep us posted on her condition.
Diarrhea is really deadly for guinea pigs.
Maybe you can get another guinea pig to keep the other company. I have no idea what her exact age is, but she's little. I was told that the new one is also a baby, but she's a bit bigger than the other one. My guinea pig has really messy hair. He is rather fat too.