Period on time???

So my periods faithfully begin on the eighth day of each month. My period has not began and I have taken 4 pregnancy tests. One was positive and the others were negative. I'm not sure what to make of this. My body is experiencing a few symptoms that are typical for me in early pregnancy such as frequent urination, headaches, tired all the time, bloating, lower abdomen feels hard. I put all my cycle information in and it says that I have a 3% chance of pregnancy and that my period should start on the 15th which is way off course. Any input is very welcome!!! Thank you!