Losing weight.. Help?

I'm losing weight a bit too fast I think and idk what I'm doing wrong? Isn't it bad to lose too much in a week? I've lost like 4 pounds and I heard that's bad. But I mean I eat up to 1200-1400 daily and I exercise and eat back calories so how is this even possible? And I exercise like kickboxing class, or I go running for about an hr.