"This post is being edited in another window" No it's not!!! But I keep getting this message

I keep getting the message that my post is being edited in another window when I am writing in draft mode on my blog. I am definitely NOT editing in another window, but I am forced to refresh the page every time, sometimes losing material and sometimes being able to save before refreshing. This has never happened before and just started yesterday. I can't get rid of the message until I refresh the page, which slows down writing and causes all sorts of problems with saving my work. Is this happening to anyone else?

I would like to fix this, but honestly it feels like a bug in the system. Any help with this matter would be greatly appreciated. I'm attaching a screen shot so you can see the message.

Thanks, Barbara