Hi. Thanks for your query and an elucidate history. Noted that you are 18 years old, fit and and in good health. You have pain in the testes on alternate basis on sitting and lying down but not on standing or during sports. This does not come in cancer, never on both the sides as you have thought. Cancers are always painless inthe initial period. I would advise ou the following: Get an ultrasonography and color doppler test done to see the probable diagnosis which can be: - Varicocele - epididymoorchitis Clinical evaluation by a general Surgeon or Urologist would help to correlate the findings and to plan for a treatment if necessary. Till then apply scrotal support. Drink plenty of oral fluids.