Why does my penis get hard around my girlfriend? I'm 9 years old and whenever I'm around my girlfriend my penis gets stiff, raises, and I can see it through my pants. Is this normal? Is this a disease or something?
Your penis getting stiff is called an erection. You are near the start of your body changing from a boy's body to a man's body and part of those changes involve your penis and scrotum. Early on anything, everything, and nothing in particular can cause you to have an erection.
What I would like you to do is start reading about the changes you will be going through and what those changes are for. See:
What you are experiencing is not a disease, but the normal process of becoming a man. It will take about eight years for all the changes to take place.