The high estrogen level increases the level of prostaglandin, which begins to act aggressively at a time when progesterone is in deficit, and that may result in spasms and stomach aches.
Estrogens also stimulate the central nervous system, and this effect is manifested in greater irritability and anxiety. Progesteron usually works as an anti-anxiety hormone, and therefore its deficit in the common days of PMS increases irritability and cravings for sweets.
The important thing to remember is that food can actually help you deal with the symptoms.
If your most bothersome symptom are the mood swings, you should reorient your diet towards legumes and complex carbs rich in vegetable fibre, B vitamins, and magnesium. Eating such foods will increase your consumption of tryptophan and tyrosin, two amino acids required for synthesis of serotonin and dopamine (commonly known as happiness hormones).