A few days before my period is to start or sometimes the day of, i feel more tired than usual and irritable and weepy for no reason. what exactly is happening hormone-wise that makes me feel this way?

Premenstrual: Hormonal changes do cause premenstrual symptoms,and resultant fluid retention can cause cerebral effects resulting in personality and mood swings. In my experience taking a mild diuretic for a few days before the period can mitigate these mood swings significantly. Discuss this with your physician or gynecologist and find out if he/she agrees.