Coffee fits neatly into the receptors for a brain chemical known as adenosine, which is partly responsible for calming you down. By interrupting the activity of adenosine, coffee makes you feel awake and wired. You may think that's a good thing, but consider that virtually every study of PMS has implicated caffeine as a major culprit. The added stimulation and nervousness from the coffee makes you feel edgy at exactly the time that feeling calm would be a blessing. And the blood sugar fluctuations it produces contributes enormously to cravings.
The body treats a coffee jolt as a "stress response" much like the adrenals shooting a jolt of adrenaline into the system. Coffee also increases urinary secretion of important minerals such as magnesium, potassium and sodium and uses up a fair amount of vitamin B1. Not only that, the coffee plant itself is a virtual repository for toxins such as pesticides and other harmful chemicals. Although in the short run it may suppress appetite, over the course of a day most people find it stimulates cravings more than suppresses them.