I should have started my period yesterday and did not. i woke up this morning with blood in my panties and have continued throughout the day, though it has been light. i am also having constant lower back pain. is this my period or is it implantation blee

Sounds like period: It sounds like you just started your period one day later than usual. Implantation bleeding is not really as common as the internet would have you believe. When it does happen it's usually 6-12 days after ovulation. A pregnancy test 7-10 days after unprotected intercourse is much more helpful. Implantation bleeding occurs exclusively in pregnant women.