I was surprised at how closely your experience (and a commenter's experience) mirror my own. During my life, I've had a few periods of a few months where I focus intensely and work nonstop, and the work does not feel like effort at all. For me, it also comes with a sense of complete confidence, a feeling like I am fulfilling my purpose in life and that everything is exactly as it should be. It is the best sustained feeling I've ever experienced.

Unfortunately I've only experienced this three times in my life; typically around major life events (once when starting a new job in a new industry, once when quitting that job to make my own stuff, and once in grade school: the summer between 10th and 11th grade, for some reason). I look forward to seeing more research, and hopefully one day can apply these learnings to manually trigger this intense focus and motivation.