Why do I make less than minimum wage?

I work at IHOP, and I make $3.62/hr. Minimum wage where I live is $7.25. I often go an hour or so without any tables at all, because I work nights. I usually make about $15-20 in tips, while working around 6-8 hours per night. Therefore, I am not making minimum wage. Why is this, and how is it legal? If it's not legal, how do I go about filing a complaint?