since you had unprotected sex, if you want to cover your bases, you can get tested for gonorrhea and chlamydia now. you need to wait another 3 weeks for syphilis testing. you can test for hepatitis, herpes and hiv after the 3 month mark.
odds are this isn't a std going on but since it's been there for 3 weeks, time to see a medical provider for a proper exam.
grace
No it is not a STD. Was sex protected or unprotected? See a Dr if concerned.