ANSWER:

This is an issue of dispute. 

The most authentic opinion is that those who fast 6 days of shawwal after completing the month of Ramadan will be rewarded as if they have fasted the whole year of Fard fasting. 

Those who complete ramadan but fast 6 days not in shawwal would be rewarded as if they had fasted one whole year of Voluntary fasting. 

QUESTION: After the month of Ramadan, I made up for my missed days then I fasted 4 days of fasting of the month of Shawwal, my period came I could not fast the remaining 2 days, my question is if I regain my purity outside the month of Shawwal can I fast the remaining 2 days even if the month of Shawwal is over ?