The ladies below have given you some good advice. After having the doctor check all of this (and the ultrasound) and nothing is found, ask the doctor for an MRI. Not a CT scan.
A friend of mine kept having a backache and cramping. He kept telling her that she just needed to exercise more. She was already a "fit" woman. The pain was intermittent and went on for an entire year. Finally before a trip overseas, she was in so much pain that she went in begging for pain meds to get her through the plane ride. The doctor on call refused to give her anything without testing, and a previous CT scan had shown nothing, so she asked for an MRI, which I think he ordered just to appease her. That MRI showed some awful stuff going on around her kidney and all over her uterus. it was a webbing of some sort, and of course, the first thought to everyone who saw the scan was cancer. Two weeks later, she was in surgery with an oncologist, gyncologist and kidney specialist all in the operating room.
I am grateful to be able to say that it was not cancer. She was so very, very lucky. They had to do a complete hysterectomy and remove all that webbing from everywhere, including around her kidney, which was twice the size of what it should have been. The back pains were caused by her kidney. It took months for her kidney to go back to the proper size.
That doctor had misdiagnosed her for a year. If it had been cancer, my friend would no longer be with us.
Be on top of your health here. Get the proper diagnostic treatment so you can suss this out.