What other than pregnancy can cause you to miss your period if you are on the pill? I’ve heard health conditions but what kind? Thanks.

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I’ve had very sore nipples, am achy at times in my low abdomen and the other day had a large amt of white and brownish tinged sticky discharge.I took pregnancy tests which were negative 3 times.
I’m sure everyone asks this but could they have been false negatives,do these seem like symptoms of pregnancy?Could the discharge have been implantation bleeding?
Thank you so much!