I had this once, years ago with the first girl I ever had sex with. She ended up just sucking me off.
Don't let this phase you dude, there will be other times you guys can have sex (and there will be if you gave her a good enough time to have 4 orgasms...you dog
). It's like going soft inexplicably or coming too soon. These things happen and the important thing is not to worry about it or take it seriously.