Why did the image get stretched?

I took a photo on my nikon d90 on auto of somebody. I was holding the camera vertically so as to get the whole person in the frame, but when i tried to turn the image around via my computer, it stretched the image and made them look bigger then they actually are. Why is that and is there a way to fix the problem. I also tried using a photo editing program to open the file and the same thing occurs. Any help would be appreciated.

What program are you using? What operating system? What exactly did you do to rotate the picture? Without knowing these answers, I can;t help you.