Missed Period, Negative Pregnancy Tests

Patient: I’m a 16 year old female and I’ve missed my period. Predicted day was to start on the 14th of february, but its the 23rd and still no period.I took 2 at home pregnancy tests on the 19th and they both said negative, so why hasn’t my period come yet?I have been having some cramps and my lower back hurts, along with a headache for the past few days. Any idea what’s going on with me?

Symptoms: Cramps, Missed Period, Negative Pregnancy tests, Lower Back Pain, Headache