I'm sorry to hear of your miscarriage and complications.
While I have not experienced a miscarriage and am not sure about complications from this, I can say that I have a history of ovarian cysts and every time I have a CT scan or ultrasound, they remark that I have extra free fluid in my pelvic and abdominal area. My doctor said it is not concerning because women do tend to have free fluid, especially if there is a history of cysts. She said if I were a man with free fluid, they would worry.
Is it possible you had a cyst rupture? That would explain the pain and the fluid. Are you still having pain? If not, that could mean the cyst ruptured and the fluid is not resolving itself?
I understand your fear of surgery and your concern about the fluid. With your history, it's perfectly normal to feel nervous about this. Does the doctor seem okay with waiting a couple of weeks? If so, and you are starting to feel better, I would probably wait and follow up. However, if you are still in a lot of pain and the doctor feels the surgery is necessary, I would probably follow his advice.