my gynea told me that as soon as ovulation occurs, mensturation always occurs from 12-14 days after. This is the same for all lengths of cycles whether regular or not. If your cycle is irregular, then it simply means that the first half of your cycle is the problem, for some reason you ar taking longer than normal, or an irregular length of time to produce enough hormones to bring an egg to maturity. Most women can match up ovulation with a very mild/slight pain in the low right or left abdomen (usually right and left alternately).
So theoreticially, if your cycle is 20 days, you would probably ovulate on day 8. Presumably, your menstrual time is short also.
|The anomoly is when you ovulate twice in one month which is extremely rare.
don't knot if this helps; as I'm not an actual medical doctor, nor a doctor of any type. If you are worried, you should go see a gynea.
mimififiXX