Too much sex. I need a break!?

Ok. So ive been with my GF for over a year and love her to death. She happens to LOVE sex. Alot. So on new years she told me her resolution was to make love with me every. single. day. of 2011. At first i was like HELLZ YAAA!!! But it is now Feb 23rd. We've done it everyday for over 50 days in a row now. Im soooo tired. My balls hurt. I really need a rest. But somehow i manage to get it up everynight long enough to get it over with for the night. How do i tell her i need a couple days (weeks if possible) to recover? Will this hurt her feelings? And is this normal? Do other guys get tired of sex and need a break? Or are we suppost to be sex crazed animals that can do it every day of our lives? Please help!!!!