HI again!

Ok so took my first clomid tablet last thursday night and was all good, then on sunday morning I woke up and lo and behold I had my period
Now how in the world does this happen? I had not had a period since I went off the pill last year in November!!
Could the chemicals in the clomid make me get a period? As my gyno did heaps of bloods on me and they said i was not ovulating - hence no period.
Can anyone shed some light on this please?! As i am a bit freaked out.
The gyno has told me to stop taking the clomid on sunday (took three tablets already) and to re-start them on days 5 - 9