early period?

my period is usually pretty regular. every 26-27 days. this month my husband and I had sex every other day during my fertility window. now my period seems to be 12 days early?

my periods are usually very heavy and about 5 days. this one seems to only be for about 48 hours, and has been medium flow.

I looked up "implantation bleeding" but the blood seems too red, and it seemed to happen to close to when we had sex. I did take a pregnancy test that was negative. but it would only have been a maximum of 6 days from when we had sex.

any ideas? help. just a weird period? something more?