Guinea pig biting. HELP!?

I've had a guinea pig named sprinkles for about 2 months now. other than lately, she was a very sweet and gentle guinea pig. I've had guinea pigs before, but never had this problem. she's rather young, but kinda fat. if you go around her chin or stomach, she bites you. and sometimes even if your just sitting there. and it's not a nibble, it's a full on bite. To punish her for this, I don't hit her, but i put her back in the cage until she calms down. Also, if you take her out of the cage and sit her on the floor to run around (which I never do unless I'm with her to make sure she doesn't eat anything bad or get into trouble!). I love her so much, and I'm worried maybe she's in pain. please help! I live in a very small town surrounded by more small towns, there is no veterinary care for miles. I also put up a video of her biting. I need help! (please excuse my freaking out in the video, she climbed up on my back. XD) http://youtu.be/8r4a36QSduo <----video