Need advice.

Shyann • Mama💗😇💓

Okay here's my problem, my period is EXTREMELY consistent never late, and only 5 days long. I know when I ovulate and everything. Now, my period was due the 13th, it's the 18th and no sign of it coming.. I took 3 pregnancy test around the 12th and they were negative. SO what could be causing this?