As has been suggested, at least part of the reason could be that drinking directly from the bottle doesn't allow you to control the dose.
Taking too many vitamins/supplements can be just as bad for you (if not more so) than having too few. Taking too much iron can lead to sickness and constipation. (With really high doses it can kill you). Excess Vitamin C can also cause digestive problems. Taking too much Vitamin B6 can lead to nerve damage. (They're all in Floravital).
Doctors generally advise avoiding food supplements if you're eating a healthy diet. The only supplement which they say that everyone in the UK ought to take (particularly in the winter months, but not exclusively so) is Vitamin D, which isn't in Floravital. (One of the symptoms of a lack of Vitamin D is excessive tiredness).
However if your tiredness persists, do see you doctor, as there are a multitude of possible causes (including, for example, the early signs of diabetes).