Weird lines on nails?

I have some weird, brownish/black/purple (whatever) lines down some of my nails. It starts all the way from the cuticle and past the white part so that if I look straight at my nail so that it looks like this ---> ( I can see the discoloration. I'm not sure when they showed up, but they're on most of my nails. They're not so dark as it looks like I drew on it, but it's pretty noticeable, especially on one thumb. I've looked it up and what I'm finding is that it's either nothing or it's cancer which doesn't make me very happy... Anyway, I have a habit of keeping my nails kind of short on one hand and constantly cleaning under them (OCD) so maybe it's from trauma? I just want to know what it is and if I should go see a doctor. My mom noticed my thumb and asked if I had hurt it in anyway. When I said no, she didn't seem too concerned, but who knows. I may be stressing over this but I keep reading about people who have had their fingers cut off because they didn't catch a cancer in time, and I'm a guitarist, so I'd really rather not have that happen... Please let me know if you have any advice or info.