What are you doing to look after it at the moment?
My advice would be to rinse your mouth out with antibacterial mouthwash and make sure to clean your mouth after you consume anything other than water.
A yellow layer over your tongue after a piercing is normal, and a white circle around the area of the piercing can be expected too it's sort of like the tongue's version of a scab)...
but any green pus or discharge is a sign of infection. Does that sounds like what is happening with your piercing?
Because oral infection can be pretty serious, I would recommend seeking medical attention if that's the case. Infections are really easy to turn around if caught early, it's the people that ignore them that suffer the most.
The problem with tongue piercings is that:
a) Your tongue is constantly moving, which delays the healing process
b) your mouth is full of bacteria (I believe it has a higher level of bacteria than anywhere else on the human body)
Another pointer is to make sure your barbell is tightened on a regular basis. Always do this with clean hands (preferably washed with antibacterial soap).
Also, after the second week, you should have been asked to return to your piercer to have the bar changed to a shorter one.