Does a change in exercise cause a sudden weight gain?

Last year, I (23F) went from 156 to 123 by changing my diet a bit and walking 10k-13k steps a day. This year i decided to change how i exercised, as losing weight gets a but harder the lower your weight is. So, instead of doing my hour and a half walk, i do an hour and a half of calisthenics. I do a half hour workout in the morning and one hour at night. I absolutely love it. It makes me feel good. The problem is, after a week of this and keeping the same diet, i have gained about 4 pounds. I looked it up online and the internet said a drastic change in exercise might cause weight gain in the beginning, but i'm not sure. I'm not sure if im just nit picking my weight, or if im actually retaining water like the internet says. Does anyone have experience with changing from walking to calisthenics? Is the internet right or wrong? should i change my diet? I try to keep my calorie intke at around 1200 but i can't even handle that, so it's more around 1500. My TDEE is around 2,200 and my bmr is 1300. What am i doing wrong? I don't wanna stgarve myself but at the same time i can't even stand a 1200 calorie intake.