What Does Pain In The Lower Back On The Right Side Indicate?

Hi, My daughter has had kidney stones in the past. Recently had back pain again. Went to doctor, told her to drink plenty of water to flush out, which she did. The lower right side of her back still gets random pain. She had a CT scan and it did not show any kidney stones. She says when she drinks a lot of water at one time, that is when she gets the back pain. Her doctor told her to have another ct scan in 6 mos. Any suggestions. she feels good, otherwise and it does not stop her from her daily activities.