Can STDs show up a day or two after transmitted?

Patient : I had unprotected sex with a girl that claimed not to have had sex with anyone else, but me (some 4 months ago) and the other way around. We had unprotected sex and the next day I noticed red blisters on my thigh, where the leg joins the body. In addition, I did do some minor shaving of the pubic area a day before. Today is the second day I have them. In the morning they contained that white fluid you usually get in pimples (English is not my native, I do not know the word) and some of them had popped and are just red like simple blisters. Can an STD show up so quickly and should I be worried? Can it be a skin condition?
Symptoms:  Red pimples on thigh, where the leg joins the body. Nothing on my genitals as of now.
Images: 
Can STDs show up a day or t...-1
Doctor :   You mentioned that you had unprotected sex followed by development of skin rash. You even mentioned that you shaved the area prior to sex. You are definitely at risk of STDs, but the condition mostly appears not to be a STD. Your description of lesions mostly suggest that you are having folliculitis or boils owing to post shave. STDs rarely present with 3-4 days. So it is mostly boils out of shaving. Consult a physician to establish exact diagnosis as you require antibiotics for further management and to rule out STDs.

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