I'm not sure where you live, but you should be able to see a doctor or visit a Planned Parenthood at your age. I'm not saying you need to in this case, just that you should be able to.
Most women will go back to their normal cycle the following month (after ECP). For others, it might take a bit longer. Your negative pregnancy tests are proof you are not pregnant. Stress can impact your cycle, so try to relax. If you have not had issues with your period before taking ECP, then your cycle just needs to sort itself out.