Something that I hear all the time is “why am I gaining weight? I'm working out everyday!”, and the answer is VERY simple, and the reason why I don't like scales.
People pay too much attention to their weight, and to the BMI scale, that they don't realize that what the scale says doesn't really matter! Why? Because MUSCLE WEIGHS MORE THAN FAT.
If you're working out, you will be getting thinner (if you're not body building), but your weight will either stay the same, or you'll gain weight even! It all depends on what you're doing. If you're strength training, this is definitely true, but if you're just doing cardio and aerobics, you'll probably just see a decrease in weight.
This isn't me, but it demonstrates how toning and muscle gain works. This is “lean muscle”, which means you will look very long and lean, but you'll weigh more than when you first started and that's ok. It's normal, and it's perfectly healthy!
So when you realize you've gained a few pounds, don't freak out! If you've been working out, it's probably just muscle and nothing to worry about.