Is being on a diet making me ill?

I'm currently on a diet for summer trying to lose a dress size from uk size 10 to size 8. I have been doing it for a few days and have lost 2lbs. The diet consists of 3 meals a day, no snacks, 1 hour of exercise a day and consumption of no more than 800 calories a day. I have been getting stomach cramps, being breathless and feeling weak so constantly that the only exercise I can do is walking. Is this diet making me ill or is it another health problem?