What Does Little Bleeding After Having Sex Suggest?

History of regular periods lasting 5-7 days, with no break through bleeding. Yesterday ended period and had unprotected sex, afterwards had some bleeding. I believe this may be my period not having been 100% done. I noticed small bleeding yesterday night. This morning and today no noticeable bleeding until this evening. Where the bleeding is enough to wear panty liner or tampon. I ve been under extreme stress this weekend & sick with bronchitis. Not sure if these extended period of if it could be implantation bleeding? I believe its too soon for the second. Please help me, why am I bleeding again?