My period is now 15 days late & I've taken 5 pregnancy tests, all are negative but I have symptoms of pregnancy, for eg: exhaustion, nausea, lightheadedness, backache, cramps. Could I be pregnant? In 15 years, I have never missed a period or been more than 2 days late

You should go get a blood test that will definitely tell you.

Have you been to the doctor to get a more accurate test? If you haven't then I recommend that you do. It will put your mind much more at rest to have the more accurate test done!