The PP is correct - your scale weight is not static. It will fluctuate 3-7lbs per day/per week based on stuff that has nothing to do with losing or gaining body fat. Here's what I mean:
intense exercise, especially strength training - this makes the scale go up due to water and blood shifting within the muscle. It's normal and it's not a fat gain.
constipation/irregular bowel movements - even your body waste has a weight because everything inside you is affected by gravity.
normal fluid and hormone shifts - your body has bioryhthms related to menstruation, sleep/wake cycles, etc. Your hormones and body fluids will shift in response to these rhythms, and that can make the scale go up. Not a fat gain.
Eating fiber, drinking water - because everything is affected by gravity, these two heavy elements will have great weight inside you as well. Fiber can also cause bloating. Not fat gain.
Too much stress/too little sleep – can make things stop because your body perceives crisis. When the body perceives crisis, it secretes more cortisol which tells the body not to burn fat, but to store it.
eating too much sodium, eating processed and packaged foods - causes water retention leading to a gain on the scale. Not fat.
Any change to diet or exercise program - makes the scale go up as your body tries to work out these changes. Not fat gain.
Remember that to gain one pound of fat - ONE POUND - you'd have to eat 3500 calories ABOVE your weight maintenance calorie level. Meaning, if you maintain your present weight at 2000 calories a day, you'd have to eat 5500 calories to gain just ONE POUND of body fat.
If you know you didn't over eat, then you know you didn't gain body fat. It's fluctuation and nothing more. It will work out.
Keep track of your measurements - they don't lie the way the scale does. If your measurements shrink, you're losing fat no matter what the scale says.
See, people say they want to lose "weight" but what they mean is that they want to lose fat. They don't want to jiggle and they want to look better naked. This has almost NOTHING to do with the scale. What it has to do with is BODY FAT.
Track your measurements, take photos of yourself every 2 weeks, and if you have a pair of jeans that are too small, get them out. Hang them on your bedroom door. Try them on every 10days to 2 weeks. If you can fit in a little mroe each time, you are losing fat no matter what the scale says.
Look at it this way - if I told you I could knock 20lbs off your scale weight in about 10 minutes, you'd be really interested right? Ok, so I cut off your leg. BANG, you've lost 20lbs off the scale. Did you lose any fat? Nope, everything that jiggled before still jiggles. You didn't lose fat.
Focus on losing fat. That's done with eating healthy in a calorie deficit, doing strength training AT LEAST 2x per week, and doing cardio.
The scale is just the pull of gravity on your body in that moment. IT doesn't measure your body fat percentage, it doesn't measure how you look naked or in your bathing suit, it doesn't measure how fast you can run or how much weight you can lift. Just gravity.