Positive OPK then a negative?

I have had very negative OPKs until yesterday afternoon...yesterday was cycle day 13 and at 6pm I got a dark second line- not positive yet but almost. Then 5 hours later at 11pm I got a positive. We baby danced last night so fingers crossed! BUT this morning I expected another positive if not a test line darker than the control line. But it was much fainter and obvi negative! Is this normal? To have your surge and then 8 hours later have it be gone?