Swollen Thread Like Structure On Shaft Of Penis. Is It Due To Sex?

I have some swollen whitish thread-like irregular structure on the shaft of my penis ( almost at the middle of the shaft towards the left ). It is painless & can be shifted here & there just under the skin very easily. It becomes prominent whenever I m aroused & my penis is erect. It appeared in April, & disappeared by itself. It has now appeared again a couple of days back. I have had a steady girl friend, and we used to have regular sex for almost 2 years; the problem never occurred then. Since March this year, we haven t been able to meet because our marriage is being opposed by her family. So, for last few months my frequency of masturbation has increased. What is this problem ? What is its cause ? What should I do to prevent it from reoccurring ? Should I have sex & stop masturbation to prevent this problem ? Or will this problem reoccur if I have sex ? I don t want to be disloyal to my girl friend. We love each other, & plan to marry as things settle down in her family. Please help & guide. Your valuable advice shall be heartily appreciated. Amit Sharma