It’s either you or the day itself. If it’s the day, just wait it out. If it’s you, it’s either your self-perception that’s off, in which case some affirmations or treating yourself to even a small glowup might help; or something has gone subtly wrong, most likely sleep deprivation or dehydration. So drink some water, take a nap, use the nice hair product as a treat, and tell yourself you’re a sexy badass.