How come I stopped losing weight? i havent change my diet and workout?

I've been into kickboxing and biking as a workout to lose some weight and I find it effective, following with a sensible diet for weightloss. but the last 10 lbs is so hard to get rid. I didn't change my diet or my workout routine. what should I do? should I lower my daily calorie more and do workout more? will that slower my metabolism if I lower my daily calorie more?