Spotting?

Okay so what can cause spotting? I’ve been sexually active for about six weeks (we’re both adults), and last Sunday my boyfriend and I had sex. We’re using spermicide and also condoms. On the following Tuesday, (two days later) I had some spotting that night. Just a little bit of blood. And nothing the next morning. So I was thinking okay, maybe implantation bleeding? Because my period was still two weeks away. But since then I have taken a pregnancy test that was negative and also gotten my period. Just wondering what could have caused the spotting? Is it because my body is new to the penetration? Please let me know🥰