This has been happening for about 6 years. Now and then (maybe 3 or 4 times in 1 month) I get a sudden tiredness like all my energy levels drop and I have to put my head down for 10 minutes. Normally I'm fit and active, and I have an on-the-go job and attitude to life in general. After 10 minutes I bounce back to 100%, it's like it never happened.
I've started having a sugary drink or carbs to perk myself up when it hits, but it does unnerve me that it happens at all!