Why do I gain weight eating at maintenance 6’1 185 lbs

I recently lost a considerable amount of weight around 40 lbs and now I’m trying to lose body fat but build muscle and strength. I have been extremely consistent with my calorie intake at around 1800 and it works well but I have noticed almost no progress in the gym or in muscle growth. Today used a online body recomp calculator to try to see how many calories I should be eating to achieve my goals and the numbers were very high 2500 on workout days MINIMUM and 1900 on rest (which seems reasonable). Today I ate around 2,100 calories but my weight has increased almost three pounds. I know it’s probably water weight but it still scares me and from past experience it seems like eating like this will put on multiple pounds a day. I just doesn’t make sense as to why I see so much weight gain from LESS than my maintenance. I already feel like I’ve been stuffing my face and am struggling to get more calories in. Any advice??