A few weeks ago I noticed a red lump on the shaft of my penis (around 5mm diameter), I dismissed it as nothing, however it's now gotten slightly larger and has turned purple. I'm 16yo and not sexually active, I'm planning on going to the doctor if it doesn't disappear soon, however I was wondering if anyone could tell me what it is.

Just go to the doctor so that will put your mind at ease. Don't squeeze it or pop it.

I agree with you going to the doctor. I am not a doctor nor a professional in this area, but can tell you that repeated masturabation, like a couple times a day, can help a small bump appear. It has happened before to others, and goes away with decreasing the times one masturabates. I am not saying this is the case with you, but it's a thought. Good luck.