Hi DaisyB and others living with CFS
Daisy it is quite usual for the crash following physical or mental exertion to take 48 hours to kick in and take you by surprise. I used to run a small forum for people living with CFS and almost all of them reported the same thing, relapse after 48 hours following exertion. My worst relapse was after my mother died. I had helped nurse her and care for her for her final 8 weeks. I just kept going, running on adrenaline as you do. The ensuing crash and relapse lasted 8 weeks and started after her funeral. Nowadays I can keep things on a more even keel but I am still rubbish at saying 'no' and getting myself into bother

I do worry about the effect of returning to a stressful job for you but does your HR department know of your difficulties?
I also have sensitivities to a lot of ingredients in toiletries and medication. Basically the longer the list of additives the worse the effects will be. I bought some senna tablets this morning and was offered the cheaper, chemists own brand ones. When I got home and checked the leaflet there was a list as long as your arm of long names with a lot of 'm', 'x', 'y' and 'l' in them

I went back and changed them for the Senokot with a much shorter list of ingredients.
CFS does seem to lead to a chemical sensitivity for many people.