I was attacked 2 hours ago, so my shield should have covered me for another hour from now.
Yet, I was attacked again while my shield was up because my points dropped again and my shield now reads 2 hours.

I can only assume someone did a revenge attack against me. Is this how the mechanics work?

When ever i get attacked while my shield is active, my shield resets to 3 hours. I have never lost points while i have an active shield

How does the revenge attack work, then? It doesn't decrease your points?
I can revenge attack someone at any point, assuming even if their shield is up.

Sorry if this has been covered before, I'm just trying to understand it better. Not sure if I had a glitch or a delay in things showing up or what.

If you have an active shield it doesn't decrease your points but will increase theirs. It also will not show up in your battle log.
But if you get revenge while your shield is down you will lose points and find the person in your battle log.
Same applies if you revenge someone.
There could be a delay/bug because i have been attacked from the future before.