NEED ANSWERS

So my bf and I were having a little sexy time, taking our time with fore play and such and then I notice about 20 mins in he is shaking like not as bad as shivering but like when your muscles are weak kind of shaking. My bf is a marine so there is no way his muscles were weak. When I asked him if he was alright or why he was shaking and if we needed to take a break he just said it’s because he just is so horny. I tried looking it up but I have no answers. Anyone

else had this happen??