I wish it were that simple, but it's not. We have to keep in mind that our weight is made up of a lot more than just fat. There are multiple body composition models used, but the one I recommend most people use when trying to lose fat is a 3 compartment model that consists of the following:
Fat
Muscle
Bone
I recommend this one since it's easy for most people to visualise those 3 components. Keep in mind however that roughly 70-75% of muscle mass is water. Why is that important? Because sometimes we can be fooled by the scale into thinking we're not losing fat, or that we're gaining fat when we're not. Most likely, what you're seeing is simply changes in water weight.
A really common scenario is for people on low calorie diets (like most people here), to see a plateau and think that means they need exercise more and/or eat less, but when they do that, they actually gain a bit according to the scale. The reality is they didn't gain fat, they retained more water.
I think it's important to keep in mind that we all have something called a Basal Metabolic Rate (BMR). This is the minimum number of calories your body needs simply to stay alive. It can be thought of as the number of calories you expend per day even if you were completely sedentary.
BMR is a complicated subject, but on average, the bigger you are, the higher your BMR. Yes, it goes down as you lose fat, since fat is not completely metabolically inert, but fat loss does not have nearly the effect on BMR that losing muscle does. This is one of the main reasons bariatric patients are told to focus on protein intake because protein is needed to help prevent excessive muscle loss when dieting. More muscle = higher BMR = faster weight loss, or being able to eat more at goal weight
The average BMR for women is ~1400 k/cal per day. Higher if you are taller or more muscular, lower if you are shorter and/or have lower muscle mass. Men, for obvious reasons tend to have a higher BMR that's more in the 1700 range.
My point in telling you all of the above is that it's highly unlikely that someone eating 900 calories a day needs to eat even less if weight loss has stalled. Frankly, if that is the OP, then I'd actually recommend exactly the opposite: try upping your caloric intake a couple hundred k/cal per day and see what happens. I know it seems counter intuitive, but your body is not a simple machine where the calories in vs. calories out paradigm actually works.
Happy to discuss more and provide additional info, but this post is already overly long.
Best of luck.