i have constant back pain from the least effort
i'm not overweight

You didn't tell us if you are pregnant or not...
Anyway here are some tips about back pain treatment:
The most frustrating aspect in treatment of back pain is that there is no "magic solution."
Most individuals recover completely by simply avoiding strain to their spine.
Patients often find help from ice, heat, and medications.
If the basic steps do not alleviate back pain, the next step is to seek medical evaluation. Depending on the symptoms and the length of the problem, your physician can properly organize a treatment schedule.
You should visit the doctor if:

Your back pain persists beyond a few days

Your back pain awakens you at night

Your have difficulty controlling your bowels or bladder

You have a fever, chills, sweats, or other signs of infection

Any other unusual symptoms

i think my muscles are weak ,it's not severe

i think you should do (back exercises) to strengthen your back muscles,

thanx julia u're right my brother told me so
i began to do it
thanx alot