Every once in awhile, Pink Diamond would enduce sleep. It was a great way to access her subconscious to solve problems. Unfortunately, her subconscious wasn't something that was conciously controlled. It was a trade off, that was why it was so great at problem solving. Didn't mean it always addressed problems she wanted to solve at the very moment. In fact, curled up among her work in her sanctuary, she was plagued with a terrible nightmare.
Being a side sleeper, her gem was projecting some terrible images of some specter onto an opposing wall. It would manifest itself to be Umbra, from her perspective, towering over her. Though, being a gem of raw emotion, the projected images exhumed the overwhelming sensation of a much larger threat behind the viewer. It was unseen, and it was a force she hadn't encountered yet: the Prince of Oblivion.