Another ovulation question....

So my cycles are usually about 28 days give or take a couple days and this was my first cycle on clomid 50 mg days 5-9.  I had a + OPK and BBT confirming O on day 20 of my cycle, yet I still started my period on day 28.  Just wondering if anyone had experience with this or a clue of what I should do???

That happened to me ...Then they up'd the clomid to 100 with Metformin and I got a +OPK on day 17 and got preggo with twins.  Did they do the day 21 blood work and an ultrasound?  Talk it out with your Dr.  Blessings to you.