Yes, as @sb4 just said, straining does stimulate the vagus nerve....so it might be something to do with that??
And right -blood pressure is another consideration. How is your blood pressure normally?
If it's a bit low that might raise it enough to make you feel better. Low blood pressure can cause some yucky symptoms. Take a look at that maybe.
Thanks @sb4 and @Wolfcub I have hypertension instead of the hypotension that most people with ME have. I think the vagus nerve stimulation might be the reason. Also could it be that the parasympathetic nervous system is getting activated ? whatever be the reason doctors simply dont understand anything that I say.