HI. You have a typical history of flank pain while swimming. As you may be aware of , flank muscles are most active during swimming and must have been sprained causing you the dull ache. If you do not have any other symptoms which you have categorically denied, I would go for spam of muscle and advise you the following: Rest and avoid any activity which can enhance the pain more. Take a muscle relaxant, anti-inflammatory medicines and apply rubrafacient ointments locally to give warmth and soothing. IF you develop associated symptoms of any nature consult your Doctor for clinical examination and treatment accordingly .