Abdominal Pain

So I've been with my boyfriend for 3 months now. We just recently started having sex. As much as he hates it, I make him wear a condom and pull out. I started my diet back after taking a month and a half off and I'm taking these pills called phentermine or addapex. Now I know diets can screw you up on your periods as well, as it did mine the very first month I started these pills with my diet. I started classes back again too and just have been stressed with it. I've been having pains in my sides and stomach area above my belly button, almost like dead center of my stomach and have been having a hard time going to the bathroom like I'm constipated.  My period is due today but still haven't started. Should I be concerned? Or is it just the pills and diet again? What could the pains be?