You need to talk to INZ, to sort out your situation. It is much better if you tell them yourself that you have just realized you have broken the rules, than if you wait for them to find out, or for someone else to 'helpfully' inform on you. To hold a second job alongside the one that is named on your visa is often possible, if you get a variation of conditions to allow it - INZ will explain what you need to do if you want to continue and if the situation would be legal (if the second job does not compete with your main job that is on the visa).