Is it normal to get an erection when my pretty female doctor examines my penis?

I am 15 and go to a private doctor once every few months about my bladder problem for a check up. My doctor is female and is around 35 and very very attractive. Every time i go, she makes me take off my trousers and boxers. she then with gloves on pulls back my foreskin and asks me to cough . but the thing is, as soon as she sees or goes near my penis, i have a full on erection, let alone when she touches it. My mum stands there but does not watch. The doctor just smiles. Is this normal, when she pulls back my foreskin, i feel lick i wonna *** which i know is really bad. Is what i do right. I cant help it, as much as i try. is it normal?