Reasons for missed period?

So this is my 3rd question today. It follows my other two questions. So my boyfriend and I have been sexually active since my last period. I am on the pill and he always "pulls out" so we dont use a condom, and yes I am realizing how stupid it was....

Anyways I missed my period and it raised some concern because my period is ALWAYS on time plus the pill so regulate it. So I took a test. It said I was not pregnant. Sooo, why would I be missing my period, if not prgnancy, then what? It has never happened before.

the pullout method does not work
sperm is in pre-c*m and no man can control that
so if you use it your likley to get pregnant
as well as at risk of getting an sti or std
its one of the lowest effective things you could do to prevent pregnancy
reasons for missing a period are
the pregnancy test you took was wrong,
there not 100% effective so you could in fact be pregnant
if your over-exerscising too much
stress...if your extreemly stressed
and if your not eating right or starving yourself
also you could just be skipping a period
in which case you just skip one and it returns like normal the next month