It could be that you are getting tired - as you get tired, there is a tendency to play closer to the tip of the reed (because it takes less pressure) and to bite down (because your muscles are tired). Both of these together will make your reed is close up. Your reed will feel very wet, because you've been playing for a while (which is why you would be tired).
Take a five minute break - when you come back, you should find the problem has gone away.
It is possible to 'overplay' a reed during a single session, but generally it takes hours...and the reed was probably ready to die anyway.