Welcome to Health Care Magic.
I have gone through your query.
Based on the description give by you the pain must in the right lumbar region.
Pain in right lumbar region is commonly due to Appendicitis
, muscular strain, urinary stone
or infection or intestinal spasm
As far as your case is concerned, the possibility of appendicitis is highly unlikely as the pain due to appendicitis shall be much severe.
I would like to go in favour of muscular strain as you are not experiencing pain while sitting and only during activity.
Taking muscle relaxants or Tramadol
tablets shall solve your problem.
If you still have doubts and doesnt want to take a chance, get an Ultrasound
abdomen done . This would clear all your doubts.
Hope you got the solution.
Wish you a very healthy life