This morning I woke up with an erection as many man usually do. I undressed and went to the shower and noticed a very pronounced and somewhere sore vein on the left side of the head of my penis. I have never ever seen a vein there, much less popping out like that. (I didn't even though the head of the penis had veins like that. Anyway it's rather sore and it is obviously much less pronounced when not erect but still visible. What is this and is it serious?