Have you been tested for Celiac? It's a blood test. It could be many things, what other tests have been performed?
If the constipation is still persistent you can get some magnesium citrate at your local pharmacy, I had to take this before because nothing else would work. You can't take it on a regular basis but it will get you through the weekend. It will definitely flush you out and make it easier to stand, etc. (when I get constipated I can't stand up straight either!).
Drink lots and lots of water, the magnesium citrate will dehydrate you.
Could be, since ovarian cysts have been found, you may have a tendency to get cysts in general, and any of them can get pretty large, so perhaps you've got one that is blocking up your digestive tract. The black stools can indicate bleeding, so could be things have gotten a little out of hand in your abdomen. I don't know when your surgery is scheduled, but with you having a fever and possibly blood in your colon, it's a good thing you're seeing your doc Monday... they might want to move that surgery up a little quicker, but that doc should know if it's an emergency or not.
Stay in bed all day today, take your temperature every few hours, try to have someone come around and visit you or at least talk on the phone with you sometime today. If your temperature rises above 104, go ahead and get a ride to the emergency room. But it might also be a good sign that you finally had some digestion, even though it was painful and didn't look right.
Let us know what happened after your doctor visit tomorrow. If your doctor sends you home, but you once again have pain and fever like you did Friday, then definitely go straight over to the ER at that point. Fever and signs of internal bleeding, along with a history of cysts, is nothing to fool around with.