As an independent contractor, your work was done pursuant to a contract, or agreement, as to the terms of employment, what you would be paid, etc.--even if it was only an oral (or verbal) agreement, it was still a contract. Neither party may add to or change the terms of the agreement without the agreement of the other party, so she may not charge you any fees which you had not agreed to. She also must pay you for the work you did, pursuant to the terms of your agreement, and may not withhold your compensation. If she will not pay you voluntarily, you can sue her to recover your money. A good option is likely to sue in small claims court, where you can act as your own attorney.