Hi there, since you mentioned that your pain is not constant, and only sometimes, but it is quite severe and causes dark colored urine (tea colored? bloody?) I would be most concerned for a kidney stone. Kidney stones classically cause very severe, intermittent pain in the flank like you are describing. Most tend to pass on their own but can be quite painful, and I would recommend you see your primary care doctor for an additional workup as he or she may opt to get some imaging of your abdomen and to prescribe you with pain medications while it passes, since sometimes kidney stones require referral to a urologist for surgical intervention if they are very large. I hope my answer is helpful to you and that you feel better soon.