thanks for your query.
In this day and age, a 5 month pregnancy
cannot go undiagnosed.
it is quite simple - visit your gynecologist and have a check up and an ultrasound
You would know within a day instead of waiting for months.
it is highly unlikely that a pregnancy test would be negative for so far along.
ALso, it is an extremely rare condition that periods would be regular.
Most probably you are not pregnant.
What you could be suffering from is called pseudocyesis of pregnancy.
Please read this up