As an STD assistant, I can provide you with information about herpes. A small cluster of blisters on the penis that reappears monthly could be a sign of genital herpes. Genital herpes is typically characterized by painful or itchy blisters in the genital area, though some people may not experience any symptoms at all.

It's essential to consult a healthcare provider for proper diagnosis and testing if you suspect you have genital herpes. They can perform tests such as PCR (polymerase chain reaction) or viral culture to confirm whether these are indeed caused by the herpes simplex virus.

Remember that only medical professionals can accurately diagnose your condition based on specific tests and evaluations. It's crucial to seek guidance from them for appropriate management and treatment options tailored to your situation.