I solved this issue today and finally made the iCloud sign-in prompt go away, after just being too lazy to do anything other than click on "cancel" every time the iCloud box popped up on my screen for months. While I have a Sony VAIO desktop, I can't imagine the solution is any different if using a Mac computer, since iTunes is just another site/program accessed from the Web.
At some point, you probably forgot that you said "yes" to joining the Cloud when Apple added this feature to iTunes. Unfortunately, every time thereafter, it will ask you to sign in to iCloud, with the prompt that "your session has expired." All you need to do is sign in to iTunes and go to your Account Information. There is a section there called "iTunes in the Cloud", and all you need to do is decline/unsubscribe from this feature. You will still be prompted to sign in to iTunes for purchases, but the iCloud prompts will go away.
Also, someone on a discussion thread about this issue complained that iCloud mysteriously "stole" all of their music and put it in the Cloud. But at least as of today, you would need to pay $24.99 to store your music in the Cloud. So it's unlikely that iTunes did this on its own, since it appears to be an upgrade they require people to pay for and subscribe on an ongoing basis.
Hope this helps!