Spot on my lung...

So, I was just wondering how likely it is that the spot on my lung, that my doctor just found, is Cancer... I have smoked now for about a year and a half, and have had a constant cough, with stuff that comes up, for about 2 years. I'm overweight, and almost 21 years old. No lung cancer has run I'm my family, but my grandfather died of colon cancer. My doctor said he wouldn't worry just yet and to re x-ray in 3 months, but its all I think about, and I'm a bit scared... Anything will be helpful, Thanks!