I have lost 0 pounds in the past 18 days but I have lost an inch off my waist (dont keep track of other measurements)
how is this possible?
I havent been exercising!

well that answered my question. because exercise builds muscle which weighs more than fat, but if you aren't exercising. are you using the same set of scales?

Gut contents. Sounds gross, but that's what causes a lot of day to day shifts. If you change the way you eat, you change the proportion of solids to liquids to --I hate to mention it-- gasses in the stomach and intestinal tract. That can make a big difference in the waist measurement.

It can also fluctuate a lot depending on where you are in your cycle, partly because of water retention. I'd guess that right about now you're somewhere around two weeks out, therefore you've got the least amount of water retention for the month.

When you're measuring yourself, I have found it's best to take the hip and thigh measurement, along with calf and bicep. That gives you a better overall picture of how you're doing, especially if you write the results down and "keep score" from month to month. It usually takes me about a month to show a real decrease that's not just temporary. And it's better for your ego to measure after your period, or in the middle of your cycle. You don't need to think about water bloat, lol!

well hate to make this more confusing but:
- I use the same scale and weigh at the same time of day once a week
- I don't have a menstrual cycle due to depo provera so don't expericne those type of fluctuations anymore

The scale and the time of day have nothing to do with it. The scale is basically measuring what you have been eating and drinking over the past 48 hours or so. It also shows -- excuse me for being explicit-- what you have or have not eliminated.

So, for example, if you eat a lot of bran cereal and salad, and then get constipated and gassy, your waist measurement might be greater. OTOH, if you starved yourself for a day or two, drank nothing but water, and used a lot of Ex-Lax, your waist measurement might be less. (Not recommended!) And since you don't eat exactly the same kinds or amount of food every single day, the body won't be the same from day to day.

And even a birth control bill that eliminates your period is not going to make every single day exactly the same in terms of the hormones your body is making. The endocrine system is extremely complex, and there are literally hundreds of hormones and related substances being secreted by several different glands in your body 24/7/365, not just in your ovaries. No matter how "even" the depo provera makes your system, it is never going to be exactly, precisely the same from day to day. That's not the way living systems work.

Some people even retain water when the weather changes, or their joints swell in cold, damp conditions. Your body is in constant, dynamic change as a living organism responding to it's environment, and everything you do, everything around you, has an effect.

Sometimes there doesn't seem to be anything you can put your finger on that would have caused the body to change. Believe me, it happeds a lot. In any case, as long as you don't have any negative symptoms, there's no reason to pay much attention to short-term fluctuations. Like I said, keep records and keep to your program, and you should see some long-term positive results. Don't worry about what happens in 18 days, worry about what happens in 18 months, or 18 years. Good Luck.