I undergo excessive diarrhea, i took erythromycin together with metronidazole simultaneously, i feel headache dizziness and so bad, what should i do? was i overdosed myself?

It is not possible that taking erythromycin and metronidazole simultaneously is what anybody would cause overdose. Both could have side effects; and I hope you did not drink alcohol. It is more important to find the cause of the diarrhea so appropriate management could be instituted. It is also important that you have adequate fluid intake with electrolytes preferred like in soup.