Losing 6 kilos in one year may not be a series problem if that is the only thing going on. I don't know how much you weighted before so I cannot tell exactly if it is healthy or not but Always remember losing 5 percent from your body in just six month without any diet or physical activity is not healthy. Sometimes Losing and gaining few pounds throughout the year is normal. I mean if you were stressed or if the weather wasn't comfortable for you, if you had I work/school load on the past year, that would trigger your body to lose weight. According to some studies losing 30-50 pounds(upto 22 kilos) a year is healthy if it is not accompanied by other symptoms of illness. So don't stress out that much but carefully try to see and remember if you had other symptoms like: fatigue or tiredness that prevent you from doing your day to day activities, lack of sleep or sleepiness, loss of appetite, reduced concentration, mood changes, blurred vision, heavy headaches, shortness of breathing, excessive dizziness, irregular heart beat... if you have seen those symptoms on the past year or if you are seeing them now you better go and get checked. It might not be serious but it might also be symptoms of serious disease like: anemia, low blood pressure, stress, anxiety, diabetes or symptoms of other diseases. So if you suspect any of these or having discomfort about your weight loss, you should go to general physician and get checked.