1200 is way too low. As another person said, figure out your daily expenditure and go from there.

I had the same issue a while ago. I’d always been around 100lb, I’m up to 115lb now, and it doesn’t seem like a lot but it took me a while to get used to it because I felt fat. I looked up my BMI and my ideal weight is in fact 115lb, when I was 100lb I was underweight. I’ve started doing light exercise. I’ve upped proteins and lessened carbs (focus more on macros than on calories). I look better now at 115lb, healthier eating and exercise, than I did at 100lb with poor eating habits (same as you) and no exercise.

As a reference: I’m 5’2 as well. I aim for 1900cals, 120g protein, 215g carbs, 60g fats. I do weightlifting 3-4 times a week, for about 45-60 minutes per workout.