My cat has started going to the bathroom in very wierd places.....WHY?

I have a two year old female that is pretty good for the most part. She has had accidents before but have been my fault for leaving the litter box too long. Now she has started going to the bathroom on my kitchen counter. I took her to the vet thinking that she was trying to tell me something but the vet gave her a clean bill of health. What other reasons could there be??????

Is she spayed? Is there a window nearby? Could she be stressing out over a stray hanging around outside somewhere? Has there been a change in her/your life that might be stressing her out (new significant other, new pet, home renovations, etc...)?

Yes she is spayed. It was done at a very young age as I got her from my local shelter. There hasn't been any change in the home. I do have three other cats but that never seemed to cause a problem before. Her habits haven't chaged. She is still very cuddly and she eats well. There is a window but I live on the top floor of a house. I am so very confused as to why she is doing this. And its only when i'm not home. I come home to find these wonderful little treats. Is it maybe cause she's mad at me. HELP