How do you deal with a controlling mother?

I have to go to stay with my Mom for two months this summer as her regular caregiver will be away. Dont know how I will take it. She tells me what to wear what to eat that I am underweight (I am the correct weight according to Dr charts) that I should not go to the gymn cos I may get germs etc etc I usually visit for ten days and am worn out at the end of that time! I wrote her a letter saying I did not want comments on my clothes, diet. She responded "I never tell you what to do!!" She also plays guilt card a lot.