Flat Penis When Laying Down?

Does anyone experience their girth decreasing when they are laying down? Specifically their corpus spongiosum?

I lose 1/8” of girth when I am laying down on my back. I have no trouble getting an erection when I and standing or sitting and maintaining one, but as soon as I am on my back my CS is non-existent and my glans shrink in size.

Anyone have any advice to fix this? This has something I have noticed before PE, I am only 24.