After teaching/talking nonstop at work without food and drinks, i felt very tired, weak and pain on the lower left side of my breast. a heart attack?

Chest pain: Any acute chest pain should be considered a heart attack until proven otherwise. In other words, if this is a known, recurrent problem, and you know your heart is ok, then it's probably ok to watch. If you don't, then you should go to an ER to check it out. Since a heart attack can lead to sudden death, better safe than sorry. Hope this helps!