I would first ask why you lost weight? It sounds unintentional. If you feel this is a significant amount of weight loss (ie. I don't know where you were starting from), then I think I would want to know why this is occurring. There are several factors (stress, depression, hyperthyroidism, medication use, dietary changes, exercise patterns, illness, etc.) that can result in or be associated with unintentional weight loss. As you can imagine some are more serious than others. A fairly good indicator of the severity of weight loss in young women is whether it is affecting there menstrual cycles which tend to be uniquely sensitive to significant weight loss.
Once you fell comfortable that none of these underlying conditions are leading to weight loss you can explore ways to gain weight (a truly rare request, believe me).
A visit with a dietitian would be a great start and probably provide the best strategies for healthy weight gain. There are medications that are sometimes used off-label to gain weight but in most if not all cases the side effects are significant in the short and long term. I would steer clear of those. Most of these medications are indicated only in extreme conditions (AIDS, cancer wasting, severe burns, etc.)