Advice needed?

I am used to having irregular periods. Some months I don't have one at all. But now I am sexually active and I had a period last month, a few days after the last time I had sex. Is there any possibility that any sperm, if there was any from precum since he pulled out, could have fertilized even after my period? 
I'm going to take a pregnancy test, but I'm just curious if it's even possible before I invest the money.

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