Previous readers have given good suggestions. Muscle does weight more than fat. Other things to remember are that water stored in soft tissues fluctuates for reasons that are both in and out of your control. For instance, we retain more water in high humidity, but also when we eat foods that are high in sodium. It is common for label watchers to look at calories and fat content, but don't forget to monitor your sodium intake. Some sodium is good for maintaining healthy equilibrium, but too much is counterproductive.
Always weigh yourself on the same scale and if you weigh yourself at home and at the MD office, be aware of what the difference is between the two so you are accurately monitoring weight fluctuations. Always weigh yourself at the same time of day (preferably morning before you eat or drink anything) and are wearing nothing. Weighing yourself nude eliminates fluctuations caused by varied articles of clothing.
If your weight fluctuates between 1-5 lbs regularly, it isn't crucial and is likely a result of things such as what you've eaten, water retention, muscle gain/ wasting, and what you're wearing (if you don't weigh yourself nude). In other words, don't sweat it. If you are fluctuating more than 5 lbs, there could be something else at play and it might be wise to consult an MD or a nutritionist.
For more info on healthy diet and metabolism, check out this hub: