He cant stay Hard....?

Me and my Boyfriend have tried to have sex a handful of times now, sadly he cant stay hard. He says he has never had this problem so he cant figure out whats wrong with him. He has mentioned though that im super tight...like its hard for him to get, even painful, but once hes in he stays hard for about half a minute and then it goes soft. Afterwards he stays soft for the most part, could this be my fault by being I guess too tight? Or could something else be contributing?