swollen penis

Recently I had sex and two days later i noticed the underside of my penis was swollen down the right side right under the head of the penis.  It is really noticeble during an erection and almost appears that a gland underneath the skin is swollen. It almost feels like a vein running from the side underneath is hard. The swelling has gone down alot during the week, but the vein is really hard. I have NO discharge and you wouldn't know anything is wrong unless you mash and feel the hardness of this knot or vein.  I am totally freaked out. Did i damage something or have an STD?