Hello, I engaged two times into giving oral sex to a guy. I used condoms on both times I gave oral sex. I understand that using condoms reduces the risk for STI. What concerns me more is that my lips could have been into contact with the uncovered part of the skin like the base of the penis and the scrotum.

Performing oral sex on a male is a relatively low risk event and the use of a condom makes it virtually no risk for most STIs (including HIV, gonorrhea and chlamydia).   Furthermore most people do not have STIs.  Theoretically, contact of your lips with parts of the penis or scrotum which were not covered by the condom might have led to acquisition of HSV or syphilis IF an obvious lesion were present.  Without a lesion there was close to no risk.  From what I see there is no medical or scientific reason for testing.  ---