Why Have I Missed My Menses?

I am over a week late on my period. I am NEVER late. My period fluctuates between 20 to 26 days but i am never late and the only time I have ever missed my period is when I have been pregnant. I have hypothyroidism but am taking medication for it. I have taken 3 pregnancy test over the past two weeks and all were negative. Please help me figure out what is going on. Thanks