I feel this too. I suspect its a byproduct of all the context switching I find myself doing when I'm using an LLM to help write software. Within a 10 minute window, I'll read code, debug a problem, prompt, discuss the design, test something, do some design work myself and so on.
When I'm just programming, I spend a lot more time working through a single idea, or a single function. Its much less tiring.