Why am I not starting my period?

Marley

So back in December I was supposed to start on the 8th. I didn't start until the 24th. During those two weeks I took multiple pregnancy tests, probably about 6, all come up negative.. now in January I was supposed to start this past Sunday on the 21st now it's almost the 26th and I have not started yet. I've taken two pregnancy tests this week and they both came up negative... What could be causing the delay in my period? Before anyone suggests stress, I need to mention that I don't hardly stress about anything.. is it possible I could still be pregnant but the tests just aren't working for me? Has this ever happened to anyone else???