Lump on arm above my armpit. No idea what it is.

Okay, so I'm sixteen years old.
I have a lump on my right arm, above my armpit. No idea what it is. I don't know if you'd class it as non-moveable, it moves with my arm, if that makes sense. So, if I roll my arm around, if moves with my arm.
Anyway, it's never gotten any bigger from when I first found it.
I also get breathless easily. It goes away when I sleep, and takes a couple of minutes or so to come back in the morning.
I know these are both symptoms of cancer, so I'm very scared. I think about it all the time, it's always in the back of my mind. It's almost as I've accepted that its cancer, and that I'm going to die.
I also have social anxiet and moderate OCD, so I was wondering if there was a stress element coming into the breathlessness thing. I also am prone to panic attacks and get scared and panicky very easily. Like..even if I'm home alone, I start thinking about horrible things which could happen, and get myself all worked up.
Anyway, I have no other symptoms of cancer. No tiredness, no losing appetite, no bloody excrement or anything like that.
I just really want some advice. Some comforting advice.
Many thanks.