Veins protruding inward when feet are elevated

I was sitting with my feet elevated high up and noticed that all the veins on my feet, which are usually protruding outward markedly when standing, were deflated so much that the skin was instead protruding inward along the veins. I've never seen anything like this before.

Does it suggest an alteration in elasticity of the blood vessels?

It looked and felt like blood continued to circulate normally. Just that a lot less of it was in these veins compared to usual. Is this an easy test for blood pooling?

 

The deflation of your veins when your feet are elevated is probably just caused by gravity, I would guess.

 

Yes, but is it normal for them to deflate tso much that the skin protrudes inward? If it's not normal, what does it tell me about my illness?