How many days are we talking since the dieting's started. If it's been less than 10 days, it's probably too soon to be able to pinpoint a direct cause.'
I think that fatigue can be the body's response to a sudden and drastic calorie drop. It's the body's way of adjusting to the lower calorie level if it were a sign of "famine." Decrease food needs by decreasing caloric needs.
Still, I think it's probably too early to make guesses, because there are also so many other possible explainations, some of which have absolutely nothing to do with weight loss.
I wouldn't be too concerned yet. If you use a food journal, saving a spot to write how you feel physically and/or emotionally, and even how long you slept at night and even naps, can help you see patterns, but it takes times for patterns to become obvious.
I noticed too, that when I started losing weight I became more tired. I think it was just my body's way of adjusting to the lack of caffeine/sugar/etc that it was used to running on. I feel better, sleep better, and the tired feeling is gone now after a month.
I'm eating better now, taking a multi plus an iron and fish oils, just to make sure the fatigue wasn't anemia again.
Keep an eye on it, track the pattern. But, if you're concerned - never hurts to ask your doctor.
One thing I notice when I'm active, eating well, and off caffeine, is that I'm more sensitive to my sleepiniss (and alertness), in much the same way as I become more sensitive to my hunger and satiety. I think it's just being more in tune with your body. Feeling especially sleepy at night might be analogous to finally learning what hunger feels like after a lifetime or many years of always stuffing yourself.
Sleepy?
Lastnight I slept about 10 hours. I never need that much sleep. I am typically great with about 8. 
