Why is my penis SO small???

Hi, basically I am 16 going on 17 very soon, my voice broke at 12, I have pubic hair, my balls have dropped, I have underarm hair, and everything has happened in puberty except one thing. My Biology teacher said the penis generally starts growth a few months after your balls have 'got larger' which means dropped and pubic hair has developed. That stuff happened 4 1/2 years ago for me, I am 6 foot 4 and have pretty much stopped growing now.
My penis, honestly, is one inch flaccid, and barely passes 2 inches erect. In my swimming lessons the other guys have big bulges in their speedo shorts, but I have an absolutely tiny one.
I have had a girlfriend for 3 years, who I really like, but recently she has been really presurising me to show her my ****, which she has a right to, because she let me see what shes got if you know what I mean. So basically I showed her it and she was shocked at how small it was, then she got over the shock and giggled for about 5 minutes. She said her 6 year old brother probably had bigger **** than me, and she also said she didnt think a 2 inch erect penis was long enough to impregnate her, which we want to do in a few years cos were so close.
I dont know what to do, she wants me to go to the doctor and see whats wrong but theres no way Im doin that. Its just SO EMBARASSING that when she wants to do stuff, I pull down my trousers to show her a penis not much bigger than her fingernails. PLEASE HELP,

[blaaaaah]

hope you have some luck with that. your girlfriend should be there for you every step of the way. dont be embarrased to see a doctor , thats what they get paid for .

[JamesStubbs24]

But its too embarrassing to do that seriously