It's probably a blood blister caused by biting your cheek. There is a tremendous close-to-the-surface blood supply in the mouth, so if you nip a blood vessel, this can keep it going a bit. Keep an eye on it, but they can last quite a while.
You should get checked out by a dentist. They are trained to recognize the early signs of leukoplakia, which is mouth cancer. If you frequently bite the insides of your cheeks (some people do it by habit, others do it at night while sleeping and don't realize it), then it's possible to develop it over time.
If you smoke or happen to use chewing tobacco, the chance of mouth cancer increases. It could be from biting your cheek, as other suggested... keep an eye on it. If it doesn't seem to improve soon then it's best of have it checked out.
You can get mouth cancer from biting your cheeks? surely that cant be right
I'm not sure that biting your cheeks will actually cause cancer.. it may be one of those things that doesn't help your chances if something else is a possible cause.
I remember reading a thread on this site about a woman who had a bad habit of biting her cheek when nervous and her dentist told her she really needed to stop because it could lead to cancer. It was a very vague summary of a conversation, so who really knows what he was actually getting at.
It seems possible that if there is a chance of mouth cancer, then having open sores won't help prevent it.
The reason why biting the insides of your cheeks can lead to mouth cancer is because, for someone who does it very frequently, they are constantly distressing the skin tissue. Over time, with constant irritation and a constant having to heal, the skin tissue inside the cheeks begins to change and mutate. That's where the possibility of mouth cancer begins. Although it's not an immediate threat, it's one of those things that has the potential to develop over time. That's why it's adviseable for someone who does it frequently to try to stop it as much as possible.