Speaking as a guy, I feel the pain of a girl being ‘too tired’ from time to time. But I mean, yeah, it’s natural. We all get… tired.
Other times, of course, you might not REALLY be THAT tired, but for…. uh… strategic reasons, you might feel it necessary to state that you are.
And to be fair, advance warning is always appreciated. Just to mitigate the potential disappointment. And as far as a credible warning is concerned, there are few more compelling sources than the T-Shirt you are wearing.