Fertile days after period?

Can you be on your fertile days just after your period?

I took plan b 4 months ago and my cycle has become Very irregular.

My last cycle was 20 days. And i had unprotected sex on the last day of my period(the 7th day...on 28 june) i took plan b again just in case.

is it possible? eve tells me today it’s my first fertile day, but idk how accurate this is.