To give a little background on my issue, I am 18M and 200lbs exactly at 5'9 (5'10 on a good day). I live an active lifestyle filled with yardwork, mountain biking, swimming, tennis, you name it. Anyways, I've had a diet change going for a couple weeks now that I thought would work for sure. One day I will eat 100 or less calories, and then the next day I will eat 1600-1800 calories. I have been told that 1800 should be at least 200 calories below my maintenance, especially with how active I am. Somehow though, I am still at 200lbs where I was three weeks ago. I drink a ton of water but especially on days where I am having very little calories. It seems that I'll lose a pound with a day of fasting, then gain it right back the next day when I eat below my maintenance still. This is confusing me and I'm just looking for an explanation as to how this could be. Also to ease any concerns that I would be eating above 1800 calories on day two, I can assure you my calories are tracked well. Thanks.