Implantation? Or?

My AF was 3/10 on 3/28 I spotted once and had sex that night. On 3/29 the bleeding got slightly more mixed with a mucous and I barely filled a panty liner in a day. it would stop and go til 4/1 when it stopped completely my period is supposed to come 4/7. pregnancy test is neg. what could this be? this is as heavy as it really got.