My lower back hurts i don't know why ?

Low back bain: Certainly there can be hundreds of reasons for low back pain and without knowing your history it is impossible to narrow down. The most common cause, especially in your age group is muscle spasm and weak abdominal muscles. Provide more info and the doctors here may be able to shed more light.

Core Exercises: Typically core strengthening exercises are the best to help relieve pressure and forces in the spine. I would try and google some or look them up on youtube to see descriptions. If you can do them yourself, wonderful otherwise ask your doctor to get a referral to a good physical therapist for further assistance. If still hurting after that i would see a pain/spine specialist for full evaluation