Are your cycles shorter than 27 days? If so, it’s 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. Usually, it lasts 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).
CD1 - CD 6: Period
CD7 - CD13: Fertile window + Ovulation
CD14 - CD26: Luteal Phase
If you have a short cycle and are TTC, it is all the more important to start tracking your basal body temperature (BBT), cervical mucus (CM), and/or take ovulation tests. Some women have shorter than average luteal phases - and the only way to know for sure is to track your cycles more closely.