Struggling with Flare-Ups and Fatigue - Any Advice?

I've been off work for the last three weeks due to burn out and flare up of my conditions. I woke up this morning feeling really good, well rested even, which is not something I have felt for as long as I can remember. However, by 3pm, my hayfever picked up and since then I've felt like I've been hit by a train... it feels like my blood is on fire and I'm wiped out. I did much less than I wanted to today, only thing I did this morning was go to the GP for a blood test and then get a coffee, I also ate properly today too - it was after lunch I just started to fade. Does this happen to anyone else? Does anyone have any tips or suggestions for what it could be? How can I go from right as rain to completely wiped out? I notice I get worse fatigue and pain after eating sometimes but then I also find the "blood on fire" feeling gets better immediately if I eat, generally this is the cade, but sometimes this happens. It doesn't seem to matter what I eat...