Help: Why am I not loosing weight its been over 2 months and not a single pound lost!!?

Well, im 5`8 and i weigh about 132 pounds and im turning 13 in a few months and ive been riding my bike for about a mile daily and eating small portions and its working for me. Ive been doing this for 3 months and i have lost about 18 pounds. And if you work out to much it wont help at all because your just going to gain more muscle which will make you weigh more over time.