I've seen that happen as well...low numbers in the PM and higher numbers in the AM because of no shot in the PM before.

Has Squallie been eating as much as normal lately? If not...that might account for the lower numbers in the PM???
Cats DO sleep a lot but, if this is out of the ordinary for him, it might be a few things.
One...cats tend to sleep
up to 19 hours or more, out of a day, spreading their waking hours out over the entire day and night. This is especially true if there's nothing different going on or interesting to keep them awake.
Two...cats can tend to sleep when they're not feeling well so, that tummy upset could have him sleeping until it completely subsides and may be lowering appetite (IF that is the case).
But, if at all concerned...and perhaps, just to put your mind at ease more...perhaps, a call to your vet if he can't see you and explain what you're seeing happening might have your vet reassure you that right now...there's nothing to be concerned about.
By the way, when cats are in pain or have infections etc., their numbers tend to go UP more often than not. So, the fact that he's had one day of being a bit more tired, with no other symptoms, may mean that he's just having a "sleepy day".