If you have a kidney stone stuck, you are likely to know about it. The stones are solid lumps, often with a high calcium content (think limescale clumps). They cause no problems when small or sitting in the kidney. But they cause excruciating pain if they get stuck moving from one of your kidneys, down a tube called the ureter, to your bladder. Most people describe the pain as coming in sharp or cramping waves, causing them to double over until the spasm passes after a few seconds or minutes. This is when your ureter is blocked but this muscular tube is trying to force the stone down by squeezing. You'll feel this from one side of your lower back, round to the front lower part of the abdomen. It sometimes radiates to the vagina, testicle or tip of the penis. This is called loin-to-groin pain. Stones then have to move from the bladder via another tube, the urethra, and out in your urine. This may correspond to more painful spasms as the urethra contracts.
You are likely to be unable to concentrate on anything else with this type of pain, so call your doctor or get to the emergency department for an assessment and they may arrange a scan and do blood tests. They will also consider other causes of pain like this, such as a urine or kidney infection, appendicitis, pelvic inflammatory disease in women or prostatitis in men, or simple severe lower back pain. While you wait for an assessment, you can take paracetamol or ibuprofen (if just for short term relief). Any stronger pain relief can be prescribed by your doctor.