Feeling dizzy when excersizing.

I try to lose weight and mainly do aerobic excersise. On weekdays i do 20-30 mins of high intensity aerobic workouts daily and i mix up 3 days a week 20 min abs,obliques and thigh and butt workouts.
On weekends i usually do a 20 minute high intensity aerobics in morning and a 50 minute in the afternoon. But i keep feeling dizzy and sometimes feel too weak to follow through the end of the workout. Today i almost fainted and i don't know why. My calorie intake is somewhat low but if i go any more than 800 cals per day i cannot lose a single pound. Any ideas what could be the problem?

[Gulfstream2]

Wait, are you eating only 800 calories per day? Or is that your caloric deficit? If that's what you're saying (the amount you consume) I must admit, that is a WAY too low, a child needs more than that just to sit around. Without knowing your weight or any of your fitness stats, I can't say what you should be eating, but in general, most people require 2,000 calories per day for their basal metabolic rate.

If you don't eat your BMR, you will feel tired, short of energy, not up to the task of anything strenuous, etc. If you too little (the quality of nurtition is paramount) and exercise, you will just burn yourself out more and more.

[Vondalynn]

You are doing too much. Your body needs at least one day of rest, plus you are expending more calories than you are consuming. Your body is telling you to change something. I would suggest seeing a Dr about finding a good balance. Not eating enough food is dangerous your body will take fat and nutrients from your organs to function properly.