Muscle weighs more than fat—so despite what the scale says, you're still losing weight.
You've amped up your workouts to include weightlifting, but you notice the numbers on the scale slowly creeping up instead of down. Must be because of all that heavy muscle you're putting on, right? Matthews explains that you should be still be losing fat, and it's unlikely that it will be "replaced" by weight in the form of muscle. So if you are gaining weight, it's more likely due to the extra calories you may be consuming (consciously or unconsciously) to recover from your workouts. Beware of calorie-packed protein drinks and bars marketed for gym-goers, and refuel with healthy snacks like low-fat chocolate milk or nuts.