It hurt my back too. Constantly leaning over with a curved spine as I look at porn and bend over to wank is horrible for the back. After a session of wanking I've literally pulled my ribcage off of my lower abdomen that it was stuck on from being bent over for so long.
Remember that the promise of porn and wanking is a lie. It will make you feel like shit and leave you empty you will regret doing it. You are better off not doing it.