If my first, second and third trimester ultrasounds, all match up perfectly to my lmp, can i be sure it was a normal period that i had and not implantation bleeding? i suffer from chronic anxiety, please help clear my doubts?

If serial ultrasound exams all are consistent with your last period, the "due date" for a term pregnancy will not be more than a week or so earlier than a date predicted from "inplantation bleeding". If paternity is the issue, DNA testing after delivery is the only certain way to determine same.

The due date, even if calculated perfectly has a leeway of 2 weeks. What doubt are you concerned about?