Where about's in the UK are you?
The reason I ask, is it an area that is say a bit more damp at this time of the year, if the problem has only just started.
Is this only when you re-sand you get a problem, if yes might be worth wearing a small dust type mask, one that just covers your nose and mouth, Agree with what Steve wrote, worth getting a check-up to be safe. I wouldn't have thought it was the fumes, from your machine, as you are using it out in the open. If it is fumes from the machine, might be worth getting a longer hose to put a bit more distance between you and the machine? Be interested to know what your doctor says/thinks.
Good Luck anyway