Since you have not mentioned about sexual intercourse, protection or so, it is difficult to give details.
As a general rule if you have unprotected sexual behavior then only there is a chance of any STD
If not so then this can be a small boil or so.
So do not worry if there is no history of unprotected sex.
Also get the blood STD panel done so that proper treatment can be done if there is any positive finding.
If all these tests are negative then why to worry.
If anything is positive then you should consult our Doctor or a Specialist for clinical examination and get treated accordingly.
Hope I have answered your query. Let me know if I can assist you further.
Dr. T Chandrakant, ENT & General Surgeon