You are currently working on a contractual basis....Therefore you presumably have a contract with this company that stipulates duties and pay scale. I would request a meeting to talk to your employer about maybe drawing up a new 'employment' contract based on a commensurate salary in line with clearly defined duties. If they are happy with your performance thus far they will most probably be willing to negotiate with you on this issue.

Be kind to yourself hvert...and Good Luck.