What Causes Swelling On Penis?

hi.. i am 27 years old male soon going to get married..i had a problem in my genitals last week. I got a swelling on the shaft of my penis(just below the head of penis) and this swelling was throughtout the base of the penis. I consulted a doctor which told me that there is nothing to worry and gave me some medicines. after that d sewlling was gone. i want to know what was d reason of swelling..?? I am very tensed please help.