Morning anxiety

This morning I woke up with anxiety, seemingly out of nowhere. I sat with it for a few minutes, and eventually it dissipated. All is fine now. However, I’m still unable to figure out what thought may have brought about this feeling. I really can’t think of anything that might be making me feel anxious, even now as I sit here and try. Is it definitely a thought that caused this feeling in me, or could it be something else? Can i really credit morning anxiety to thoughts, even when it seems to appear before thoughts start to take hold? Is it subconscious thoughts, maybe?