A total stab in the dark here but the timing makes me think it could be related to a hormone shift. What are your temperatures both during and before/after the tiredness hits?
I’ve been excessively tired this last week due to what I now think was inadequate caloric intake (definitely unintentional and I’m remedying the change that resulted in it). Do you get any cravings during this time that you’re not satisfying and/or do you notice any change in your appetite that may result in undereating?