There are reasons why the scale could show a gain: water retention from not drinking enough water (or eating too much salt) and also muscle gain. If you're weight training, it's best to not go by the scale and instead take measurements and go by how your clothes fit. For example, I started seriously weight training in January. I have lost only 2 pounds since January 1st, but I'm down a pant size and have lost 10 inches. The scale is a liar!
This is not me, but an example of someone else. See how the scale can be a liar?!
If it's such a liar, why do we give it so much control over us? I wish I had an answer! That number on that blasted little square on our bathroom floor can ruin your day before it has even begun. It can make you feel like throwing in the towel because all of your hard work doesn't seem to be paying off. But you know what? That scale doesn't show you reduced cholesterol levels, reduced risk for heart disease and diabetes or the years you've added to your life from your healthy changes.
If you're a daily scale addict like I was, I'm challenging you to try something new: put the scale away and only get it out once a week to check in on your progress. Start measuring yourself once a week too. Always weigh (and measure) at the same time every day. If you weigh one week in the morning and then the next week later in the afternoon, you will not have an accurate judge of whether you've gained or lost.
Also, I hate those BMI calculators as much as I hate the scale. I'm 131 pounds and according to the chart I'm overweight! Those BMI calculators don't take muscle into account. So yes, BMI calculators are filthy liars too!
There are many ways to tell you've lost weight even if the scale doesn't show it. Loose clothes, more toned physique, inches lost, more stamina and just feeling better overall. This week I have realized I need my wedding ring resized because it has almost come off a few times. 18 months ago I had to use soap and water to get it off my finger! Celebrate the little things and don't obsess over the scale. I'm not saying it's not a good idea to weigh in occasionally. Just don't let it be the ONLY way that you measure progress.