No, there is no risk here.
The skin on your stomach is too thick for anything to penetrate. When people talk about skin to skin contact for infections like herpes, it's mouth to genitals, genitals to genitals, or genitals to anus, all unclothed.
The skin on your genitals and mouth are thinner mucus membranes and are far easier for germs to penetrate. Some infections, like gonorrhea and chlamydia, need to enter your urethra, by the opening at the end of your penis.
As you already suspect, no STD would give you symptoms this quickly. As for anything else, I can't imagine anything giving you symptoms that quickly, unless you had a reaction to the soap you used.