It could be something called luteal cyst. I'm sure you know that when you ovulate a follicle in your ovary ripens and ruptures producing an egg. This follicle then turns into the corpus luteum, which releases progesterone to maintain your womb lining in preparation for a pregnancy. It will continue to do this until around 12 weeks when the placenta takes over. In a normal non-pregnant cycle, when the corpus luteum doesn't get a signal that there is an embryo it breaks down, stops producing progesterone and you have a period. Sometimes though it develops into a cyst (not like a PCOS cyst) and it keeps releasing progesterone that keeps your body from having a period even though you're not pregnant. Apparently this is the most common cause for very late periods with lots of BFNs in between.
Here's a link to POAS that describes it well https://www.peeonastick.com/hptfaq.html#18
I hope that you just ovulated really late and get your BFP, but it might be this luteal cyst, in which case you might have to go to a doctor to clear it.
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