I just recently had a baby in August well in September I had bleeding for one day and I go to the doc in October and they said I'm pregnant I asked what the bleedin was and they said it was probably implantation bleeding so how do I know what my due date is

If you're pregnant and bled for one day, it was not a true period. Bleeding can happen in early pregnancy for several reasons.

Your due date is based on your LMP (last menstral period). If you ovulated, had sex and became pregnant, that first period after giving birth would not have occurred.

If this is the case, you'll have to wait until an ultrasound to determine your estimated due date.