What Causes Pain In Penis While Masturbating?

I was giving my boyfriend a handjob and he kept saying how much he liked it but then when I could feel his penis start to pulse I would tighten my grip and thrust harder/more intense and he would make this breathing noise through his teeth and at one point he grabbed my hand and put it on his balls. I think he was experiencing pain but I'm not sure. Do you know what caused this or what exactly is going on? How can I prevent it from happening again?