I'm certainly not one who supports taking 'sickies'. I've only had 3.5 days off in the last 35 years (i.e. an average of 1 day every 10 years) and I've just finished a stint of working 13 hour shifts despite the fact that I've got shingles which is leaving me feeling shattered.
However, it occurs to me that, if you don't get a few days away from work soon, you might well end up taking far more time off if you make yourself really ill. (That won't help you or your employer). Speak to your doctor and get him to 'officially' sign you off. (Even though you don't think that you're physically ill, you might have a virus which is 'knocking you back').
Please note, however, that the advice in the previous paragraph is a case of 'do as I say, not as I do'. I've not even registered with a GP since I moved house 17 years ago ;-)
Chris