High fertility 5 days after my period?

I'm in my first cycle TTC and have been using clearblue advanced since my period ended 5 days ago. it's shown low fertility as expected until this evening which is showing as "High". is that even possible only 5 days after? my cycle is about 31 days although last month was 37 as it was 6 days late, perhaps thats confused things? any insight welcome!