Been loads of 'bugs' about - I've suffered all December. I think many of them (throats, colds, coughs, stomach etc.) may not have dreadfull symptoms at the time - you can go to work through them, but the actual virus (or whatever) remains in the body and takes quite a while to shift. This can drag you down and feel as you do, about 70% of normal.
I have found the only answer to be rest, and if you do excersise (run or walk) to make sure you drop back as soon as you start to labour and sweat up, keep it easy. I do loads of stretching and gentle walking and jogging, weights at home to keep limbs functioning but without stressing the lungs.
I have never had a GP agree to prescribe antibiotics, they always told me the same thing 'it's just a virus, rest and it will go away'.