Normally have extremely heavy periods, the last two have been so light I didn't need a tampon. Have no appetite & feel sick. What could it be?

Observation. Stress, weight loss, medication, exercise, pregnancy, menopause and other benign things can cause menstrual irregularity. If your cycle does not resume a normal patterns soon, a visit to your gynecologist is suggested. If you've had unprotected sex recently, take a home pregnancy test ASAP.