What Causes Lower Abdominal Pressure With History Of Ovarian Cyst?

my 23 year old daughter has had pressure in her lower left abdomen for the past 5 years, since she gave birth. She had an ultra sound, and nothing abnormal showed up. Although, she had been diagnosed with an almond sized cyst on her left ovary during her pregnancy, or shortly after giving birth, I can't remember. But she continues to have this uncomfortable pressure, off and on, for the past 5 years. Could the ultra sound have missed something?

Thank you!