Gained back the weight I've lost

I used to be at 107kg but I dropped to 104kg which is my weight where I stepped at the scale last week but this weight I checked my weight and it's back to where I started at 107kgI don't get it and it feels quite frustrating since I worked hard for it. I count my calorie and I don't drink anything but black coffee without sugar and water before my eating period I suspect that it's muscle since at my job I move heavy boxes 4 days a week for 8 hours a day but lately since it's a more busy period I've been lifting more heavy and more often like 15 to 20 boxes weighing about 25kg to 30kgbut I don't get how I can gain 3kgs in a week, have I been slipping up and eating more? via /r/intermittentfasting https://ift.tt/xS8gtCm https://ift.tt/dEJRDcU