I will not advice you take analgesic pills immediately at this juncture. Muscle cramps and injuries during playing may recurr when the whether is bad or cold. Before any playing one need warm up to reduce the risk. As you have said that the begins with your swimming trainning, I guess it may due to faulty procedure of the training session. Avoid that exercise or type of the training which particularly gives you pain but continue with the other.
Some of the measures that will help lower back pain are given below-
1.You should undertake aerobic exercises that do not strain the back such as swimming, walking or jogging.
2.Flexibility and strengthening of muscles: Performing abdominal and back muscle exercises strengthens the muscles and exercises like stretching, yoga, tai chi increase flexibility in the hips and upper legs which results in proper pelvic bone alignment.
3. Use seatbelt and avoiding diving in to shallow water can help in reducing the risk of lower back pain.
4. Stand smartly maintaining a neutral pelvic position. People standing for longer periods should alternate placing each foot on a footstool as it results in taking some of the pressure off of the back.
5. Sit smartly by choosing a seat with good lower back support, arm rest and a swivel base. Place a small pillow behind your back to maintain its normal curve.
6. Lift smartly by lifting from the knees and not from the back. Keep your back straight while lifting heavy objects. Avoid twisting the back while lifting.
7. Sleep smartly by sleeping on a medium-firm mattress. Pillows are also helpful if they do not force your neck up at a severe angle.
About your diet, it should have all necessary nutrients including vitamin D, magnesium and calcium which reduce the risk of osteoporotic fractures and help to stave off arthritis.
Quit smoking, if you are smoking as it results in diminished oxygen levels in the spinal cord thereby affecting the healing process in case back muscles get strained or injured.
Try to reduce your weight as overweight puts your back muscles under strain which increases the back pain.
Above all these simple body mechanism which help to reduce back pain, you should undertake various back pain exercises under the supervision of their physician or a personal trainer to ensure a proper and effective workout program.
Lastly, you need to be diagnose with a medical practitioners to check any other possibilities which aggrevate your lower back pain.
"Wish you a speedy recovery".