exactly what Birdgirl said I was gonna tell you...it has happened to me a few times in my lifetime...almost everytime it was me psyching myself up to make sure I do good and oh shit...what happened..nothing..pretty embarssing and can snowball because it will even get more built up..which makes it worse..
it is best just to chilled with the situation, relax, good foreplay and man what a difference that makes...plus knowing what it is makes a big difference in how you deal with it.