Reference letter woes ?

I interned at a company for several months of which I had no oppertunity to ask my boss for a reference letter because she went on vacation unexpectedly. I emailed her asking if she would mind writing me one and she replied that she would mail one to me by a week or two. It has been almost two months (knowing that the publishing industry is very busy in the month of September) but I waited to ask again. How can I ask again politely without coming off as agitated that she forgot about it?