What Causes Abdominal Cramps And 'fluid In Endometrial Cavity' On An Ultrasound?

I just received results of my transabdominal transvaginal scan. It states there is a small amount of fluid/blood in the endometrial cavity. I also have a left ovarian cyst measuring up to 1.4cm and currently have an IUD. I have been having abdominal pain cramps and bladder pain for the last 3 weeks. Tested to see if I have a bladder / or urinary infection and that checked out ok. My OB says I'm fine and wants to send me to a Family Physician. I also am on Humira for Psoriatic Arthritis. Can you help determine what is the matter?