Hey Carolina, it could be how your working out or maybe your diet..what kind of workouts do you do now??
Had that problem too! Im 5ft11 and kept jumping up and down between 48kgs and 53kgs for years. It was my diet and not enough proper exercise and not enough proper sleep.
Well I had the same problem before. It took me alot of help from different trainers. Basically u have to eat proper food to gain weight as well as proper workout is needed to tone up ur muscles. Try to eat food that contains proteins. Please mind *food that contains protein and not much fat*. Atleast it is what helped me gain proper and stable weight. Also stable muscles. Hope it helps u out.
See your doctor about it. They may have suggestions. Just to rule out a medical issue. If not you need more protein in your diet. Protein is the building blocks of muscle. And maybe eat a bit more often. Snack during the day. Eat things with good fats and protein in them...and some extra calories. You are spending more than youare taking in hence, the weight loss.