Particularly without knowing your past history, age, weight, fitness level, etc it is impossible to know what is going on... but a 'ureteral stone' (a kidney stone that is moving down toward the bladder) is a possibility. Other possibilities include a pulled muscle (there are a number of small muscles in that part of the body that can become injured without noticing when it happens), disc herniation, arthritis of the hip joint (that has become aggravated for some reason), appendicitis or diverticulitis, GYN illness (are you a woman?)... As I have written in the past, there is a process to try to identify the cause of the pain. One of the first things I would do with you is see if the pain is affected as I manipulate the hip joint; if it is, I would look toward muscle injury or arthritis; if it is not, I would look toward a stone or a disc injury. If you had associated GI symptoms, I would think about diverticulitis, for example.
If you just have pain, and no other symptoms or findings (no blood in the urine, for example; no fever, no nausea, no weakness or numbness), I would treat the pain with a combination of VERY GENTLE stretching, never to the point of pain!, a heating pad on the painful area, and if there is no history of gastritis or ulcers, an over the counter anti-inflammatory agent like ibuprofen. Small muscle strains take 8-12 weeks to recover; discs take considerably longer. If I noted blood in the urine, fever, or significant numbness, discoloration, swelling, or weakness of the extremity, I would go in and have some imaging done.