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 (i.e., the amount of time that lapses between your ovulation and your next period). Usually, it lasts 14 days, +/- a day or two.
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). This is how women get pregnant while they have their periods.
For example:
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 ovulation test strips results. Some women have shorter-than-average luteal phases, and the only way to know for sure is to track your cycles more closely.