Herpes doesn't generally cause the sort of symptoms you describe; it
simplySimply sleep does not cause a diffuse redness, and typically doesn't involve the base of the penis or scrotum. You describe typical tinea cruris (jock itch). Or you could have picked up a yeast infection from your sex partner (not an STD, but sometimes men get genital rashes from women's vaginal yeast infections. Either one could have been triggered in part by the corticosteroid (medrol) plus antibiotic.
Most likely you misread a herpes information site. In people with known recurrent herpes, sometimes the first symptom of an outbreak is a red spot or patch, but within a day it develops into blisters or sores. A red rash that doesn't change over a couple of days, and that is spread out like you describe, isn't herpes. Your problem probably has nothing to do with your sexual encounter a week ago.
You could try an over-the-counter cream for jock itch (or athletes foot, the same disease basically), like Lamisil. If that doesn't clear it up over a couple of days, see a health care provider. But this doesn't sound like any STD.
Good luck-- HHH, MD