If you have regular 28-30 day cycles, then you were in the safe period when you had the intercourse. Please remember that pre-ejaculate
contains enough sperms to cause conception around the ovulation period. Implantation
bleeding occurs around 10 days after ovulation and mimics a natural period sometimes. The only way you can know is to get a pregnancy
test with a fresh early morning sample, 3-5 days after your due date if you do not get a period. If you wish for an earlier confirmation, you may go for the blood test
and the trans-vaginal sonogram. See a gynecologist for further suggestions. Take care.