Why I Am Not Getting My Periods?

Hi. In June I didnt have a period at all, I noticed that I did spot a couple days before my birthday (june 25) but no period. I did finally start on july 11th but it was a weird period I would bleed one day and then stop and then start again. I went and saw my gyno for a yearly check up they tested me for pregnancy and std tests and they were all negative now I am spotting a brown color. Could you please explain? Thank you!