Is what the questioner did—performing ghusl and prayer—correct, and what should she do after her menstrual cycle stopped for a long time then returned with bleeding for days that exceeded her usual period?

If you sought a fatwa from someone who issued this ruling to you, then there is no blame upon you for following their fatwa if they are among the trustworthy people of knowledge, for this is what Allah has commanded: to ask the people of knowledge and act upon their words.

As for what we issue as a fatwa, the maximum duration of menstruation is fifteen days according to the majority of scholars, and this is what we prefer. So, whatever blood a woman sees is considered menstruation as long as its duration does not exceed fifteen days, whether the bleeding exceeds the duration of her usual period or not.

Therefore, if the total number of days you saw blood did not exceed fifteen days, then all of it is menstruation. And if you performed ghusl before your menstruation ceased, then this ghusl is invalid, and you must perform ghusl again. You must also make up the prayers that became obligatory upon you after your purification from menstruation, because you performed them without purification, and making them up is more prudent.