I want to caution you to stick to whatever you have already told her. Regardless of if you decide it is too much (or if others thinks that), you going back on your word will be more harmful for these upcoming years. She needs to know what you say is something you will not waiver on.
I personally disagree with others here and think that your punishment is not too harsh. I love John Rosemond, http://www.rosemond.com/ , and his thinking that modern society is WAY TOO kind to our children. He states that this is why America is the way it is today and I agree with him 100%.
In his book he addresses an issue like this, it was a teenage girl who stole $60 from her moms purse. His punishment for the crime was literally striping away ever parent-given right she had for 6 months, on which the time would start ALL over again if any other major discipline issue occurred. (like lying, direct defiance, ect)
His reasoning is that if you crack down on a child once like this, you are ingraining in them that this kind of behavior is not acceptable. Your future years will be much less stressful, as they will conform to what is expected of them.
On the other hand, if you go back on your original punishment now, you are just showing her that you do not keep your word and she can wiggle out of a situation. Although Rosemond is a Christian and his parenting book is written in that text, his reasoning on issues and practical advise can be used by all, as he does not usually push religion into his advise.
I have read dozens of parenting books and this one will be the last one I ever buy. You can sign up for his website newsletter, where he sends emails once a week with viewer situations and his advice. Maybe you can even send your situation in for evaluation.
Good Luck! Remember, you are mom and you know what is best, dont let others make you think otherwise.