5th wheel problem?

So I just got emptied and rehooked to my trailer, I'm driving to the gate and put the brakes on and feel a big thud. Almost felt like getting rear ended, but it felt like it was underneath me, like under the seat. I know the title says 5th wheel, but I can't think of what else it would be. I only know what they taught in the school I'm no mechanic. I've been feeling it here and there for a while now. It hits harder when it's empty so obviously the weight plays a big part in whatever sliding around down there. When it's heavy there's more resistance and isn't as noticeable. Any help is greatly appreciated

 

All pins are locked. It doesn't just happen after a load or unload. It happens in the middle after days of not touching anything. And yes I always check the jaws. I have checked for play by rocking it with the trolley valve down and didn't really notice anything. Now that it's empty I can definitely feel it. How serious is this? Do I need to worry about my trailer blazing it's own trail off the freeway? Lol

 

Didn't really notice before when it was heavy, tried it again empty and I can definitely feel it.

 

Company truck, I'm not touching it unless I know 100% what the hell I'm doing lol