Spotting

I may be worrying about nothing, but I am wondering if anyone could tell me what may be causing me to bleed between periods. I have just turned 18 (I've had periods for around 6 years now), I am not and have never been sexually active or on birth control. I've experianced very light spotting two or three times before, but since yesterday it's been almost like the lighter days during a period. My last period was a but over a week ago, and it was unusually light. Again, I know this is likely not a huge deal but I want to know if I should be worried.