cat will not use her litter box for her #2 duties

i hope you can help me, my question is. i have 13yr old cat in the last month or so,she has been doing her # 1 in the litter box, but she is doing her #2 on the carpet. what can i do to fix this problem.
   
please help

Take her to the vet immediately for tests. A sudden change in litter box habits is often the first sign of illness, especially in a 13-yr-old cat. 13-yr-olds are Senior Cats & this is when many of the diseases of cat old age come on.
   
An old cat should have a check-up every 6 months & a full panel of blood tests done yearly. Blood tests are often the best way to catch a problem early (I speak from experience).
   
The exception to this is, did you change litter brands a month ago? If so, she may dislike the new brand, so switch back.

Aside from medical issues, a lot of cats prefer to use two little boxes, one for each. Its possible that she has just gotten more picky. I would definitely check get a check up first though.