Do I have the right to be compensate for performing duties that are not in my job description?

I work as Spanish Instructor for a multinational corporation since 08/14/2016, an I've been administrating Language Test since 12/12/2014. I administered around 400 language tests. This company has a Testing Center, and pays $20,50 for each oral test. The Learning Center pays me $10,25 for each speaking and reading test, and 20,50 for the complete speaking, reading and writing test. I believe this is not right. In my pay statement the testing duties are classified as hours of Instruction, but I have all the proves which show I've been administering Language Test. I feel that the company is violating my rights. What can I do? Thank you very much!