Occasional negatives don't hurt your business, especially if you're racking up 100 plus positives a month like you've done this month. If your sales went down it's due to simply a slow period, not due to the negative.
Anyone one who plans to sell on ebay for a while shouldn't waste a second worrying about negatives...if you're honest, provide good products and offer top notch customer service, the aim of any professional business person, you'll be just fine. Just be sure to neg her back and reply explaining why she's out of line. Just be mature, sincere and absolutely don't use any insulting words. That'll cancel out the negative in the mind of buyers.
Here's another thing to keep in mind...it doesn't take buyers long to realize that they can't lose on ebay if they use paypal. If the item doesn't turn up or if it's not as it was described they simply file with paypal and they get their money back...sometimes they even get to keep the item anyway. If you've got items people want, occasional negatives mean nothing. So if the buyers out there could care less about your negatives why should you lose any sleep over them?? I know I don't!