Is it possible that giving oral sex to a girl get your tongue piercing infected?

Of course: If she is contagious, a fresh wound in the tongue is the perfect portal for admittance of an infectious agent--but then again, so is the gingiva. Of course, if she is infected (with what?) and you are not, why are you performing oral without a dam? If the piercing is reasonably well established, normal, healthy vaginal and anal bacteria will likely not infect the pierced canal.

Yes: The piercing creates a portal for transmission of infections. Staphylococcus aureus / Streptococcus pyogenes can enter tissues through the piercing & lead to Toxic Shock Syndrome.