It's almost universal for people who have the normal symptoms to experience this.

You get up in the morning, the combination of having been mostly still all night, and your blood pressure/heartrate being low (I think that lymph flow may also be involved) keeps your inflammation down.

But once you get upright and become active, your blood pressure rises and the inflamed tissues fill with high-pressure blood which stretches them and causes your pain....

I find that my worst pain occurs around "dinner time" or early evening.