clomid question

I started clomid 3 weeks ago, yesterday was my cycle day 21. I went in for my progesterone blood test yesterday fternoon. Later last night I started bleeding. The bleeding has continued into today. It seems like a period, but it seems too early? Is this common with clomid to cause and early period? My usual cycles are 42 days on average. I will be calling my doctors office in a bit for my progesterone results anyway, but figured I'd see if anyone else is going through a similar situation.