I came so hard without warning

The other day while with my girlfriend she put on some bisexual porn “our favourite type” she was giving me a blowjob while I was watching what was going on and was already pretty turned on.
Then at one point one lead male asks the girl to come closer, she flips herself over into doggy and presents herself to him.
At this moment I imagined it being me presenting myself to get fucked, this put me over the edge and without warning I came in my girls mouth. She wasn’t best pleased that I blew with no prior notice, but there was nothing I could do to stop it.