I agree, a trip to the vet is necessary.

If you do find out it's not health related, I've heard the what you need to do is retrain her to use the litter box. How you do this is put her either in a cage with the litter box or in a small room (such as a bathroom) with the litter box. Do not let her out until she is using the litter box 100%. If she is at first confused, what you could do is when she poops, put the poop in the box. That way she gets the idea that the poop belongs in the box - a little harder to do with pee. Whatever you do, do not spank her or yell at her and then throw her in the box. This will only make her associate being yelled at with the litter box and she will start avoiding it even more.

However, first comes the vet visit. Usually one of the first signs of a health problem in a cat is not using the box.