What Causes Spotting After A Period Along With Stomach Fullness?

I ve been taking metformin for about two months now and I ve cut back on my eating. well I finally had a period on metformin . I had sex about a week or a week and a half before I started my period. it stopped i had sex about a week later, then i began spotting after another week. im experiencing lower bad pain fatigue nausea and severe headaches and a fullness in my lower stomach. What could be wrong?