Sex one day before period...chance of pregnancy?

So me and my SO had sex one day before my period started.
We used a condom but it slipped off and he started to cum a bit but he didn't noticed it slipped and went back in.
I did some research and apparently it's possible to get pregnant even if you have sex right before or during your period. (Usually with short cycles.) due to ovulation being right after your period.
My cycle is pretty regular (28-30 days)  and my period lasts about 6 days. should I be worried about some little guys lingering around after my period ends???? 

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