Next day after three pegs of drinking alcohol feeling dizziness, diaherra, headache everytime. Is that normal? I hav a fear I have dehydration.

Not normal. You have symptoms of gastritis best would be to stop drinking if continue to ha GI symptoms talk to your physician you might be deficient in enzyme that metabolize alcohol.
Alcohol effects. If you are repeatedly feeling badly after your drinking, that is a message to cut down or stop. Dizziness and headache are common side-effects of a "hangover." diarrhea is a common side effect of the toxic effects of a lot of alcohol on the gastrointestinal tract. Alcohol does lead to dehydration by increasing your urine volume as well as by the diarrhea.