This can be a complex issue, and trust me, I've dealt with the same problem many of time (I rescue and rehabilitate cats) First, what kind of litterbox are you using? What kind of litter? How often do you scoop?  Cats sence of smell is about 10,000 times better than ours, if the box is not clean, think~ would you go to a gas station toilet that has not been cleaned in weeks?  Most litterboxes from petshops and Wal-mart are too small, do not use the one's with hoods, it just traps the foul air. Do not use litter that has fragrances~ those are for our noses, not theirs,  Your kitty is showing displeasure of her box or location. Go to Wal-mart and buy a simple plastic storage box with high sides (prevents the litter from being kicked everywhere) measure the space before you go. Use simple clumping litter, I use only BETTER WAY litter it's more expensive, but I do not have to change it more than once a month, as long as you scoop well!  Other trick in keeping the litter clean is scooping it with a sturdy kitchen collander, you will get every drop!  Cat's are very very clean, so the most common explanation is that your kitty cannot stand her 'bathroom' and is protesting!  If she still continues her behavior, if possible change the location of the box, and give kitty some privacy. Please ask more if you need, I love to help cat people.