Obviously if she was a virgin, she would experience pain. It could also be the shape of her cervix. If it points upward, towards her stomach, then your penis is like shoving two wrong puzzle pieces together. Your curve is natural. Most men have a curve of some sort, you will probably hear jokes in the locker room like, "Are you a lefty or righty?", refering to the direction your penis curves. Try again, go slow, and use lube if needed.
My honest opinion? Wait to have sex when you are older, the girl will be more, let's say, willing. And always use protection.