Dehydration is possible. Beside lightheadedness and weakness, you might noticed a less amount of dark-colored morning urine with a stronger smell than usual, dry mouth, dry skin and prolonged skin turgor (skin fold on the back of the hand flattens slowly, after you pinch and release it.
Not energy drinks (no coffeine, taurine!), but sport drinks, like Gatorade (with salt and sugar, which both enhance water absorption) can quickly rehydrate you. Use common sence about how much to drink. When you feel that you excrete all water you have just drunk, well, then you know, you had drunk enough. Your morning urine should be at least 200 ml, and bright (translucent or yellow, not brown). You might also buy an "oral rehydration solution", it's available in any bigger store without prescription. Names are Pedialyte, Rehydralyte or so... they are ment for rehydration after diarrhea.