Shoulder strain. when i hand on back of waist i feel very uncomfortable front side of my right shoulder. why i feel this strange strain?

Rotator cuff injury: It sounds like you injured your rotator cuff mildly. The best thing for this is using Acetaminophen or Naproxen as needed, avoid reaching overhead or doing movements that irritate your shoulder, and apply ice when it is hurting you. Also start working on a rehab program (i.e. http://www.nismat.org/patients/injury-prevention/exercise-programs/upper-body-strengthening-program).