Why am I so tired?

So I've been dieting for just shy of three weeks now.  I'm eating healthy 1500-1800 cal a day depending on activity. I was exercising 3-5 time per week for an average of 45min each session. Since this started on Saturday it was all I could do to get threw 30 min on the treadmill  with a speed of 2.4.  And I am seeing a nutritionist so I know I am eating balanced. 

But I feel like I could fall asleep at any moment, in fact I have.  My muscles feel heavy like just before you fall asleep or if you ever had mono tired like that.  The mind wants to but the body won't.  I get enough sleep 7-8 hours per night.  A few weeks before I started the diet I had a major bought of insomnia and I wasn't this tired then.

I have a doctors appointment tomorrow for blood work and what ever else he wants to test me for......But does anyone have any ideas as to why I would be so dam tired all of the sudden???

Stats: 32/f/295 at last weight in/5'8"

You're not eating enough.  Your daily deficit should be between 500 and 1000 calories, and when I put your stats into the burn meter at lightly active (and you very well may be moderately active), it came up with 2880.

Several years ago, I tried eating 1200 calories and excercising every day for 2 weeks.  I didn't lose an ounce, and I kept feeling tired in the middle of the day.  It was my body's way of slowing my metabolism because I wasn't feeding it enough.  I'm willing to bet if you start eating more (and frequently throughout the day, every few hours is good), you'll have more energy.

I neglected to read dolphinclick's reply and went and plugged it in too : ) could have saved myself some trouble.

You should be eating a minimum of 1800 calories a day. I'm 5'4, 240 lbs, work out 4-5 times per week (cardio and strength, 1hr total), and I eat 1800-2000 cals a day, and I easily lose 1.5 to 2 lbs a week.

While you may lose weight on your current diet, your body NEEDS fuel in order to exercise. If your body feels like it's not getting enough, it will most certainly feel like it's refusing to even get up (or stay awake). My first week working out at this rate, I had the same problem. I just looked and felt like I was half-dead. I realized I hadn't upped my calories accordingly, and it helped immensely. Make sure you're getting protein and complex carbs as those will also help making through a workout.

Eat a small snack 30 min before you work out (I eat a banana), and another within an hour of finishing working out (especially if you do any strength training). I generally have some nuts. This will also help get you through your workout and make it through the rest of the day.

I'd suggest that you have a day soon where you get your full energy needs.... the full 2500-2800.  It could be that you need a big injection of energy to get your strength back.  If you cut your food intake you can miss out on certain vitamins and minerals even if what you're eating is relatively well-balanced.  So, in the meantime, a daily multivitamin with iron might not be a bad idea.

There are many things that can cause fatigue.  I agree with other posters that you should try upping your calories a little bit, and every week or so, you might try to eat "maintenance calories" - the full amount you need just to stay at your current weight.  That might be the problem.  Your bloodwork may uncover something like anemia that might be the cause.  Also, if you live in a part of the country that gets really hot, really humid, or where summer has suddenly kicked in (that's my problem right now), that can really do a number on you.  Make sure you are getting enough water, because dehydration can make you exhausted.

Your body does go through cycles, and it could be you are just at the low end right now, but usually that only lasts for a couple of days.  You may try taking a few days off from exercising, as well, if you've really been pushing yourself, and give your body some time to recover.  You might even have caught a little bug that isn't quite making you sick, but your body is expending energy fighting it.

So, eat a bit more, drink lots of water, take a couple of days break from exercise, take a few naps if you can, and see if you feel better in a couple of days.  And be sure to mention it to your doctor!

Make sure that apart from dieting that you are also taking your vitamins.  Try taking centrum or a multi-vitamin that will provide you with 100% zinc, iron, calcium....all the vitamins that you might not be getting.  Also, you should ensure that you have 3 servings of vegetables and 3 servings of fruits daily.  I am sure that these things will help you, along with the comments that the others have said.  I know that you want to see progress, but if you are consuming so much less than the recommended calorie intake then you will be hurting urself more than helping yourself.  Oh and drink plenty of water!  Hope that helps.

iron deficency also causes tiredness. It will show up in your blood work, when you have not enough hemoglobin or a better way to measure it is with a ferritin test.

I was diagnosed with it last year, but in my case it was so extreme, I was even tired when I did nothing. So it doesn't seem to be like your case, except maybe for hidden iron deficency (normal hemoglobin values, only ferritin value is low). I should mention that my sleeping quality was much better after iron therapy.

Good luck at the doctor's!