Okay, first of all, stay calm! It's actually NOT unusual at ALL to fluctuate 2 lbs. in 5 days in maintenance. Most of us in maintenance have a 5-lb "maintenance range" (example: 130-135 lbs) to account for this. Hormones, water levels, salt levels, etc. (and even whether you're weighing yourself at the same time of day, or after exercise, etc.), can all affect the number on the scale. So we only worry when we leave the RANGE, or when there's a consistent upward trend over a period of a few weeks.
Also, it does sound like you've starved yourself some days. I'm glad you're worried about that, because it's not a good thing! 800 calories is too little for an adult to eat in a day without going into starvation mode. What I would do is, take a deep breath, and then eat IN your range (not above your range, but not BELOW your range either!!) for a week. THEN weigh yourself for a few days, look at the scale trends, and see where you're at. That will be your "real" number that you need to decide how you feel about.
Also, check and make sure you've been getting your water! (And exercise.) These are keys to be consistent about. Fluctuation is normal, but letting water or exercise slip can lead to bad places. Just don't weigh yourself immediately after a workout, because the salt balance will be different in your body and you won't get a "real" reference number on the scale.
Finally, I know you're dedicated to being healthy, so I bet you've already asked yourself this, but--make sure you know WHY you want to weigh your goal weight and how you decided that number. In other words, make sure that weight is really a healthy one for your body given your height, build, and exercise level. Sometimes people gain weight even though they're getting healthier and losing inches, because muscle weighs more than fat, so the scale stops being as important as size and muscle tone. Sometimes people gain weight because their body is honestly trying to tell them "This weight is too little for me and I'm not healthy here!"
Of course, sometimes people (like me) gain a little because we ate a few extra cookies. That's normal, and as long as you get yourself back in control it DOESN'T have to mean anything. I've been in maintenance for almost 2 years, so take it from me!
Pre-South Beach Weight (August 2006): 160-165
Maintenance range: 130-135
| current weight: 165.0