How is it possible to be curvy but toned? Whenever I work out I lose weight.

Not enough information here. Toning implies gaining firm muscle, whatever curves you have are a product of your underlying musculature, how much bodyfat you store and where, and the structure of your body overall. The muscle mass and bodyfat can be changed through training and diet, structure is genetic and cannot be.

That you are losing weight says two things: You are burning more calories than you are consuming and there is bodyfat to lose.

It is equally important to train for your specific goal AND engage a suitable diet to facilitate feeding muscle and/or burning fat. You need a plan, some discipline and patience. Some expert guidance from a trainer or a physician would be a good start.