I just stopped birth control in july and i got 1 period in sept it was 3 weeks late now this month i am 1 week late is it because of going off of bc?

Possibly...: After going off of birth control, it's not unusual for a woman's cycle to be disrupted. Some one have periods like clockwork right after stopping, and for some women it can take a few cycles. The tricky part is if you're trying to conceive - knowing what's an irregular or late period, and what is pregnancy. If you're late and not using barrier protection, then i'd first get a pregnancy test.

Maybe: The birth control pills alter the body's natural hormone levels in order to prevent ovulation. If a person had regular periods before using the birth control pills, she may not get back to regular periods until after a few months. If any concerns, or any bothersome symptoms, she can call her primary care doctor or ob-gyn doctor to ask about her concerns.