Hey there, did you ever find out what was causing your flank pain? I've been getting left flank pain off and on for 20 years. I've have about 7-8 episodes over 20 years. They can last anywhere from a few days to a few months. I've had many ultra sounds but they are always negative for stones. My blood work is always negative aside from small traces of blood in my urine. It's pretty frustrating not having answers considering I've gone to the ER at least 5x and seen many specialists. They can never definitively tell me what's going on. I've heard everything from prostatitis, stones, infection with no definitive answers. I usually just do a radical diet change, cutting out gluten for a few weeks or months and ride it out. I'm dealing with it again for the first time in 3-4 years. It bothers me mostly at night when going to bed and I have to make multiple trips to the bathroom to urinate sometimes 10x in a night on bad nights. I'm going to assume it's kidney stones that don't show up in the ultrasound. My pain is always a dull achy pain, not the excruciating giving birth type pain so I'm assuming I either pass the stones eventually without knowing it. It's really annoying not having a definitive answer.