That is probably the cleansing action or more commonly referred to as "die-off". This happens when you try to get rid of the bad bacteria, it fights back by releasing more toxins, etc. Furthermore, if you do not drink enough water, the toxins cannot be expelled quickly and efficiently by the body, it builds up and causes the headaches, fog, etc.
Usually, with a good probiotic the harmful bacteria is got rid of within a few days and die-off goes away within 5-7 days!! That is the experience with users of LB12.