only the lower half of the penis becomes erect

Hi, I am trying to figure out if I have a problem here. Whenever I penis starts to become erect only the lower half of the penis becomes erect... with continued stimulous the penis will eventually become fully erect, but initially instead of ho it used to do it when I was younger where the entire penis would gradully become erect... now only the first have becomes erect then all at once the second half up to the head will become erect. my penis never used to do this.... I'm 27 now and beginning a serious relationship in which I'm afraid to show my penis to the girl because I'm terrified she will see this strange erection process... When half erect it almost looks like there is a rubberband inside my penis keeping the second half from becoming erect thats how distinct the line is.... Any one ever have anything similar happen?

Side info that might way in. I'm currently on a small dosage of valium to help control some chronic prostate pain I have had for 4 years now. I have been on the valium for about a year which controls the pain... other than that I'm reasonably healthy.