@0scar2012,
I can't say I've experienced that - but you do understand that everyone is different...and we all have different baggage, right?
Also, have you considered that she just may have had a traumatic experience that day she doesn't at this moment wish to share with you?
Or perhaps she's had a build up of stress that's not related to you?
Depression can also inhibit sex drive.
I'm sure there's other reasons that could contribute to that, that I haven't thought of off the top of my head.
If it's a one off, I don't see that it's anything to be overly concerned about. If it happens a a few more times, that's a different story.