iTunes error message

[klmc29]

I have just recently upgraded to 8.1.1 and when I open iTunes and try to play a song an error message pops up that says for example: "The song T'equila on Ice' can not be used because the file cannot be found. Would you like to locate it?"
Not all the songs do it, just some of them. Any ideas why this is happening and how to fix it?

Thanks in advance!