Re: ovulation and period
The time between the first day of your period (CD1) and the day you ovulate is the part of the cycle that varies most. It varies from woman to woman, and may vary for a particular woman depending on how regular the cycle is.
The portion of the cycle from ovulation to the next period is called the luteal phase -- that tends to not vary very much from woman to woman (somewhere around 10-16 days) and vary even less for a particular woman, e.g., mine is pretty much always 14/15 days.
So to answer your question, if you have a very short cycle (say 21 days?) and your period runs until, say, CD6 - you could ovulate as early as CD7 or 8.
However, after ovulation, the luteal phase is pretty consistent. I understand that a luteal phase less than 10 days is considered short, and may pose a challenge for women trying to conceive - as the fertilized egg needs a sufficient amount of time to implant. (Of course, if this is a known problem, it can be compensated for - using progesterone supplements.)
I hope I've answered your question.