What Causes Penis Limpness During Sex?

Hi, may I answer your health queries right now ? Please type your query here...Hello,just the other day me & my boyfriend where having sex & his penis got limp but just before we where getting ready to have sex we had foreplay on each other & his penis was already getting limp then his penis got hard again so we where having sex & his penis got completely limp,this has never happened before we have sex 4 to 5 times a day and even if he did get limp he would get it back up and we would have sex but this time he just went to sleep,I think he already had sex earlier,what do you think?