Is it normal to lose about 5kg without exercising nor eating less? I used to gym frequently though.

Maybe. So, you go to gym frequently but not exercising? I am not sure i got that right. Losing 5 kgs (~12lbs) w/out exercising or changing in diet and having no other symptoms may be somewhat hard to explain. Sometimes restructuring your eating pattern may help (eating less dinner, for example). Also, weighing yourself on the same scale is very important, as different scales may show different #s.