I think I have a cavity but dont want to tell my parents because since the economy is down we dont really have the money to pay for treatment
What you are describing could be a cavity. However you will not really know until you go to the dentist where they can do an x-ray. I can understand your concerns about cost of going to the dentist and that your parents may not be able to afford to take you.
Still, do you really feel that is your place to make that decision? If it is the start of a cavity and you do not say anything to your parents you run the risk of a simply cavity turning into a bigger issue (larger filling, root canal, extraction) If that happens it will cost your parents even more money. So in the end you making that choice on your own may in fact have the opposite effect that you wanted.
So tell you parents. If they do not have dental insurance or can not afford it. Maybe you can look up a dental discount plan that they can afford. This will be helpful in saving them real money and you getting the work that may need to be done. I have listed a link for you to check out if you want.