1st Abnormal Cycle Ever

I was supposed to start my period on 6/19, I have NEVER been more than one day late since I started having my period 11 years ago. Well this cycle I was 4 days late, I took two different pregnancy tests and both were negative,  finally got AF late in the day on 6/23 & it wasn't even heavy enough for a tampon that day, & only lasted until 6/25. Since then I have been having aches in my lower back, and just now I started bleeding again. Anyone have any idea what could be going on?