Is blood that appears multiple times after the cessation of postpartum bleeding (nifas) then reappears considered istihadah (irregular vaginal bleeding), and is it permissible for a woman to pray and engage in marital relations in this state?

The maximum duration of postpartum bleeding (nifas) is forty days, and anything beyond that is considered irregular bleeding (istihadah), unless it coincides with the menstrual period and fulfills the conditions of menstruation (its duration is at least one day and one night, and it does not exceed fifteen days and nights).

Accordingly, any blood after forty days is istihadah, unless it coincides with your menstrual period.

It is permissible for the husband to have intercourse with his wife during the days of istihadah, but not during menstruation.