I've been taking 15mg of Phentermine for 3 months now and I get sooo tired after taking it why is that?

Typically Phen. makes you energized for a set # hours (depends on each person) then you have a "crash" and goes into fatigue quickly. Your body may adjust to it but if yours hasn't already after 3 months, I would suggest for you to try a different routine, maybe use smaller dose in the AM and not add in energy drinks/caffeine/ B vitamins which a lot of people do on top of the Phentermine.