Christine,
I don't mean to be facetious, but I'm a guy....
Christine - on a more serious note, you're post states all the tests you've had....if they've given you an MRI, scoped anus/vagina, and found 'nothing,' ......you're position on your condition is probably what is going on (other than some hidden issue in your vagina)......are you near Mayo clinic, Cleveland clinic, or some major medical center that does detailed research/investigation? That's where you have to go I think...sorry you are going through this, I know you've been through enough already (from our posts several years ago).
Maybe your GI/surgeon can forward your case to one of those major medical centers and get you seen?
Let us know what transpires..