You can't do both "develop lean muscle and lose weight". Only one at a time. That is a conversation for another thread.
Your body can swing anywhere from (+-) 5 lbs in any given day. Your body will retain WATER until it adjusts to your new diet. It will quickly shed after a week or two. Stop worrying about your weight. Use a mirror and pictures to see your progress.
Try the 1800 calories for a few weeks and see if your energy levels are back to normal, then adjust accordingly. You need to get it out of your head you’re messing up your metabolism. Your metabolism will adjust to whatever you feed it. The true art in all of this is finding the balance of energy in (food) and energy out (workouts).