How can I not be pregnant if my boyfriend came in me during my fertile days?

There could be all sorts or reasons as the result of medical status (temporary or permanent) of either or both persons.

Don't think of any particular day (or night) making conception certain (or impossible). Obviously conception is much more or less likely on some dates, but never certain or impossible without surgery or some serious medical issues (including but not limited to age issues).

There are several factors to conceiving, timing is just one of them. This is why it can take many healthy couples up to a year or more to become pregnant even with good timing.