I have this brown spot on my arm?

I have this medium size brown dot on my lower left arm. Its like up to my veins. Its only like one spot and I'm 13 years old. I don't really go out in the sun, I mostly wear long sleeve or a jacket. But once in a while I wear something short sleeve. I never really wear sun screen though. It doesn't look like a freckle nor a mole. I may sound stupid but can this be like cancer or a disease? I'm like worried that more would grow. And I had this for like 2 weeks and it didn't like shriek.