I have a red ulcer inside my penis shaft which looks like having a mucus in the center. It doesn't give any pain neither is itching but when touched gives a burning or sensational feeling, just like a mouth ulcer does. I had an unprotected sex a month ago. For many days after sex, the penis head was red and very sensitive and now after a month a single ulcer has appeared. Is it syphilis or herpes or something else? Kindly guide here? Would a blood test be enough to understand the gravity of situation? Thanks!