boom gained 1.3 kg in one week

Nice one. As others have said some of that will likely be water weight/internal food weight.

But who cares. I used to like weighing myself (as well as my normal weigh-in) after drinking a massive 34oz shake. It was a great psychological boost to see a big new number on the scales, and gave me a little motivation boost (enough to keep me forcing them down my throat every day and keep them in my stomach).

But it's incremental gains over time that count. Many of those will be too small to notice day to day.

Same with physical changes. Make sure you take a 'before' picture for the same reason. When you look in the mirror every day, you won't see the changes. Look at two photos two-three month apart and the difference should be obvious.

I gained 2 pounds this week. Stupid water weight.

OP - if you gained 2.86 pounds (had to use a calc) in a week, only about 0.5 could be muscle. So yeah, it's water weight.

Checking your weight on a weekly basis for gains in muscle doesnt add up. You could be overeating holding water etc. The most incremental sign of muscular prog is getting systematically stronger on compound lifts such as bench press, shoulder press, squat, leg press, bb row, deadlift. To be honest only a rank beginner or a drug enhanced lifter will gain in excess of 1 kg in a MONTH. Keep realistic expectstions. Yout goal evrry workout should be specific. For ex. Doing more reps or increasing poundage or more sets n volume in same time. Might take cm measurements of forearms, arms chest neck waist at thinnest, waist at navel, hips, calves n upper legs for tr acking progress more precisely. Takes 6 weeks to see clear progress w naked eye.

Water, glycogen, food

I gained 2lbs from yesterday's feast. I doubt its gonna stick.