Everyone who posted already was right in some way at least, but I want to add something which for some reason I've been encountering every time I open a thread to read today. And it's that like me, most people have no idea that when the body has been stretched pretty far by piled up stresses (please check out the Holmes & Rahe Stress Scale) our ability to bounce back is not only greatly diminished but sometimes totally impaired. So, it looks to me that by increasing your exercise intensity you pushed your body beyond that limit and perhaps precipitated an Adrenal Crash. Looking back, my first one was precipitated one day by a combination of caffeine, some alcohol, an argument with my children and finally, a nasal decongestant.
For detailed information, please read Adrenal Crashes & Reclovery Cycle by Dr. Michael Lam, perhaps you (and others) will be glad you did as it can save you from future setbacks.