Check your weight again, check the scales you're using in case there's a fault with them. If you still think you've lost 10kg in a month then see your doctor ASAP. However if you don't feel ill, you're able to do cardio work, go to a gym and presumably [as you're on this forum], practice martial arts it sounds more like a mistake of aome sort rather than a serious illness. Better safe than sorry though so see your GP.