"read only" document is being modified

I just upgraded to OS X Lion and now, for some reason, every time I go to open an excel document I get a pop-up box titled "File Reservation" and it says "document title" is being modified by another user, Open as Read-Only, or choose Notify to be alerted when it is available. I however, am the only user on my computer and do not have the file open when I try to open it.

I have tried changing the permissions but that hasn't fixed the problem.

It wouldn't be bad if I could change the document and save it, but now, every time I make a change I have to re-save and re-title the document.

Any ideas on how to fix this.

Thanks