I wouldn't really worry about your writing sample score; it is my understanding that most schools don't put that much weight on it. What you may want to do is submit a writing sample to your premed committee and have them comment on your writing abilities in your recommendation letter, that's what my school does. I think that you could also look at this as an advantage. After all you know at least one of the questions the adcom will ask you in your interview!
Don't worry about it