Talk to Internal Medicine on Extreme Leg Pain

My mother has extreme leg pain. Especially joints pain. Its really difficult for her to walk now. She is 49. My bro was born 10 years back from operation. Since then she has this pain. Her delivery was complicated she was on bed rest for whole 9 months. Her uric acid is a slight high. Doctor has now asked her to go for pelvic ultrasound. What could be the reason? And why did doctor ask for this ultrasound. She tries to be active as my bro is 10 years but her joints cramps are giving really hard time. Ps she has been to too many doctors no one was able to diagnose but this doctor has now asked for ultrasound.