It could be simple Herpes.
Doesn't really sound like Syphilis though since symptoms wouldn't appear only 3 days later (though stranger things have happened - what with people having different biochemical makeups)
What you can do is go do a STD bacterial examination (gonorrhea, chlamydia, trichomoniasis, ureaplasma and mycoplasma).
For syphilis test, you will probably have to wait another 4 weeks (though you can extend that to 5 just in case) to see if that might be.
It would be advisable to go to a doctor and have them diagnose you properly.
It happened 2 weeks ago. 2,3 days later I noticed the sores (they didnt change over time), burning and nausea and I got the right lymph node pain and swelling in my right groin yesterday - after 2 weeks.
Yes, unprotected sex but just for few seconds only... What do you think?
Have you had a "risky" exposure recently?
There's a number of things it could be and the best way to find out is to have a doctor take a look.
If you think you have something sexually transmitted I would assume there's been an encounter recently which worries you. If you describe it, we can evaluate your risk and potentially rule a few things out