Root canal done 5 months ago starting to be a pressure when i push on it no heart beat just a pressure or sensitive i guess u could say why?

See your dentist: It's not unusual for there to be post operative pressure sensitivity.That sensitivity usually goes away over some time. There should be no other spontaneous pain. However, if a previously treated tooth had no post operative pressure sensitivity and now it does some time later, i would see your dentist, as something has changed. One consideration I have found is the presence of bruxism.

Get Re-evaluated: Many times after root canal treatment, latent bacteria can begin to multiply resulting in an infection forming. Biting pressure can be the first sign of that. Additionally, a root canal treated tooth can become brittle, so a fracture may have occurred causing the pressure. Consider seeing the dentist who performed the root canal for an evaluation.

Impossible to answer: I can not answer that question because there are so many reasons why this may be happening (bacterias on a dirty canal, overfilling of a canal, additional canals, fracture, isthmus between canals, a true cyst, foreign body reaction in case of overfill, recontamination from a bad filling or crown, etc, etc, etc. Please go to an endodontist asap for accurate diagnosis.