“I’m not myself today.”  It is good that you notice this.  Awareness is key.  Then you might ask whose energy are you
carrying around?  What is it that is
blocking your true nature from shining through? 
Energetic cleanliness is all-important. 
Those around you may carry quite a different vibration from your
own.  When interacting with others, if
you are sensitive, you may pick up and take on others’ vibrations and issues as
if they are your own.  This can happen
subtly, without your awareness, until you become aware that there is no known
reason for your less than positive mood or for your lack of vitality.  Practice energetic hygiene, cleaning your
energy through meditation, and focus on each of your energetic centers
(chakras), visualizing them clear and perfectly balanced.  Do this regularly, but most especially when
you are “not yourself.”