I know you feel like you are too young to have dental problems but in reality age is irrelevant because these things happen to people of all ages. You said that you had a deep cavity which means it has had to have been there for a long time. So either you are not having routine dental care or your dentist missed it for quite a long time. Either way, it had to have been there for a while. If this tooth was painful after the filling was put in, that indicated inflammation with the nerves in the tooth. The only options you had to stop the pain were to do a root canal or remove the tooth.
If the tooth wasn't infected prior to the root canal, it is now because once the nerve is traumatized in the tooth, either from infection or a dental procedure, it becomes inflamed and infected.
You may have other problem teeth that need to be checked. But it is common for one infected tooth to make your whole mouth hurt. It is due to the pressure from the inflammation that is building up inside of the tooth. There are thousands of nerve branches in the mouth that connect to each other. So when one area is inflamed for any length of time, a pain signal travels along these branches and before you know it you feel pain in other areas.
I hope I explained it better this time.
QUOTE=ashestowake;949597]I would also like to add I don't know why I would have really serious problems. Im 21 years old, I have good oral health until now, I eat a great diet I don't even know the last time I ate anything corrosive for my teeth besides citrus fruits.
Its just overwhelming[/quote]