Oh Minxie, who knows? What a game of soldiers this is isn't it?
I suspect it might be a bit of a virus type thing, lowered immune system makes us quite susceptible to all sorts of bugs and things. Best to take things easy, drink plenty, keep warm and don't get over tired. If it doesn't settle, or gets worse best to make an appointment with a GP in case it's the start of an infection.
Hope you feel better soon,
Lyn x