If your cycles are shorter than 27 days, it is not a glitch that your fertile window starts almost as soon as your period ends. 

Most women have very defined Luteal Phases - that is the amount of time that lapses between your ovulation and your next period. (It usually is 14 days +/- a day or two.)

For example, if your period lasts 6 days and you have a 26 day cycle. Then you will most likely ovulate around CD12. Since your fertile window itself is 6 days - that will mean that your fertile window will start as soon as your period ends. (Sometimes earlier - that is how women get pregnant while they have their periods.)

So:

CD1 - CD 6: Period

CD7 - CD13: Fertile window + Ovulation

CD14 - CD26: Luteal Phase

If you have a short cycle, it is all the more important to start tracking your BBT, CM and/or take ovulation tests. Some wome have shorter than average LT phases - and the only way to know for sure is to track your cycles more closely. 

Thank you.