You should go to the OB, check into whether you have Polycystic Ovary Syndrome or Endometriosis. At the very least, research them both and see if their symptoms fit what you are experiencing.
if you are still late on your periods, or some months just not coming go to your doctors, they will be able to transfer you to get a ultra scan and cam scan. i had that problem 2 years ago and got told i had polycystic ovaries (however are gone now due to being helped with hormone pill) look it up on google and you might be able to relate symtoms :)
although the doctor may still take another pregnancy test just to be sure.
Good luck :)
You should talk to your gyno. If you are experiencing pain and irregular periods you need to see a professional, it could be cysts or just stress. But you need to see your gynecologist to make sure. Good Luck!