period in advance

hey,

I had sex 3weeks (just after my periods) ago and I used a condom but I always have the fear of getting pregnant. I was very anxious last week about this. I realized just now that I have my periods... 3 days before the "normal" day.

it never happened before and I don't know if it's because of the stress or because maybe the condom didn't work and I m pregnant... help me please