When does spotting occur if you have low progestrone?

Tragickingdom

When does spotting occur if you have low progestrone? I have always been regular my whole life but this past 8 months I have been spotting (dark brown for a couple of days) some cycles a week before my period about 6 dpo. Wondering if it is due to low progesterone as I am 36 years old. My luteal phase is about 13 days. Cycles-26 days. I usually ovulate around day 12-13.