Haha, 'man flu'... How's the weather where you are? Drastic changes over the past few days? I've been feeling rather unwell since the weekends, extremely lethargic and low concentration, waking up in the morning exhausted. I don't know whether I'm right, but it might be possibly be a combination of the weather alternating from hot to cold within a day and my slight sleep deprivation.
Well done on quitting smoking. Anyway, if you are really feeling that the "cold" is starting to affect your performance during the day, best to hop over to the local GP to see if it's anything other than a flu. If it's a flu well, there's really no way around that but trying to keep your immune system up by making sure you're eating well and giving yourself plenty of rest.