The only possible STD issue is the secondary stage syphilis, but several issues argue against it. Syphilis is almost nonexistant in situations like yours (it is very rare in the US except in gay men); you would have noticed the primary stage, an open sore of the penis; and September is too soon for secondary syphilis acquired in July -- the secondary stage typically shows up at 3-6 months.
That you haven't had a skin problem in the past says absolutely nothing about the cause. Every medical problem has to start sometime. I'm sure this has nothing to do with your sexual exposure a couple of months ago.
Therefore, this information does't change the opinions and advice I gave you above. I'll be interested to hear the outcome after you have been professionally evaluated -- but there's nothing more I can do until then.
Also I'm a 23 yr old straight male and the reason this has me afraid is because I have never had any skin conditions my whole life, not even and outbreak of acne.
I had unprotected sex with a girl in our friend group around mid July.
Welcome to the forum.
The rash you describe is not typical for any STD. That it improved transiently after a prednisone injection suggests it is an immunologic problem (psoriasis, eczema, or many others) and not an infection. Of all STDs, the only possibility is syphilis -- or, conceivably, the skin rash called keratoderma blennorrhagica, which can accompany reactive arthritis as a complication of chlamydia.
I might be able to help more if you can give me some information about your potential STD exposures and risks. In the meantime, this doesn't sound especially serious. If it persists, see your personal physician or, even better, a dermatologist.
Best wishes-- HHH, MD