Hii, Does breast masturbation causes lower back pain?

Should not affect. Certainly if you were to perform this maneuver while lying on your back I can't see how it could be a problem. If if sitting or standing I can't see it inducing or causing back pain either.
No. No. The nerve going to the breast comes from the Thoracic spine (mid back). It is possible to have some sensation referred back to this area but it should not be painful. If you are having back pain you have a different problem.