I had unprotected sex with my ex-girlfriend 3 days ago... Today I discovered that I have a red rash on my inner thigh. It looks like a ringworm : red swollen perimeter and whitish center... I've been with her for more than 7 months and nothing like that ever happened. We had a break 3 weeks ago and it's possible that she went for some adventurous sex...

But is it possible that STD symptoms can appear only after 2 days ? And if yes, what is it ?

Ok, here is some info about ringworm...

Ringworm Causes - Health encyclopaedia - NHS Direct

Do/ have you suffered from any skin conditions previously?

Thank you for sharing information but I said it looks like a ringworm... I am not sure it is one
As nothing was there 2 days ago and my skin always been healthy otherwise.

Is there any STD known to appear as quickly with that kind of symptoms ?