what’s going on??

so my period was pretty late this month but I wasn’t worried because i’ve never had sex, last tuesday my bf and I were messing around naked and he just kinda put his boner near and on my vagina a few times. after that I was freaking out because I thought somehow if there was any pre cum it had gotten inside me and I was pregnant. the rest of the week I was stressing and becoming very anxious because I didn’t have my period. This tuesday I got my period and it seemed normal and my 2nd day was heavy but my 3rd day and today (4th day) are lighter than usual. I took a pregnancy test yesterday and got a negative but I was wondering what could be causing a lighter flow or if any of you had advice?