I feel more weak,fatigued after eating b complex pill.what can this signify? shouldnt it increase energy?

Yes, but if: triggered by a non-physiologic way is likely do to a psychological source such as depression. Some fatigue with being overstimulated is physiologic. Rapid heart rate, increased work makes people tired. I see you also asked about the same symptoms after stimulants, so, psychological issues come to mind. But without an exam, I can only give general information