In a nutshell, it's the morphic field.
You had a great weekend or holiday, and you were pretty sure you were well rested... but this morning you feel tired and uninspired, and totally unmotivated, or as we'd say in SA, not lus at all :)
What you're battling this morning is the morphic field - the collective energies of everyone around you who is upset about going to work.
The second aspect of the field you're at war with is the programming inherent in the field around Monday mornings... basically, as a species, we've had the idea of blue Mondays for so long that it's become a standard in the field.
What to do to combat it?
Centre and ground yourself and then say out loud three times: just me. You can also place a protective bubble around yourself to reinforce it.