I feel like a broken record cause I always recommend this, but are you taking an addictional calcium, magnesium, and potassium supplement? I find that this can help with brain fog, weakness, etc.

It is also possible that maybe it's time to up your carbs a bit. Or maybe up your calories. It's hard to tell. You're not doing anything differently? No new exercise, medication, etc.? Hmm... it may also be allergies that are emerging as spring pokes it nose in.

I don't really have an answer, I'm just tossing around some ideas for you. Hope you can figure it out and start to feel better.