Small abscess in the Cul de Sac de Douglas...that little scoop shaped bone that is at the base of the spine.
Adhesions are possible...so are phantom pains and a 'tethered nerve' or filament at the base of the spine that needs clipping...
Just throwing out some suggestions here.
I have had that knife pain on occasion, maybe once or twice every few years, usually after sitting on a bike or doing certain exercises. It stimulates the area and causes horrible pain but in my case, it subsides within 24hrs.
They might need to do an exploratory...could a tiny piece of rectal tissue been left behind?
Sharon