It sounds like you need to go over the accounts at your school, because you feel that they might be overcharging you. Look at what money has come in from the Pell Grant and the student loans, and look at what that money has gone to pay for. If the charges your school has made are valid, and the amount they have received is not enough to cover them, then it's understandable that they would bill you for the difference. But without more information about the charges, no one can say if it's proper that they've billed you.
If you cannot afford to pay the difference, then you might want to make an appointment to see someone in their financial aid office. Also, if this is a for-profit and/or private college, consider transferring to a public college that might be more cost-effective. Even if your student loans were covering the full cost of your attending college, the less you borrow the better, because those loans will have to be paid back some day.
Good luck to you!