Pcos has been ruled out after examination but unsure why i get light periods on month and heavy another month? i just don't know whether this is normal. i'm only 21?

"periods irregular" : If PCOS has been removed for consideration, irregular ovulation secondary to stress, weight gain, etc. will also delay the onset of menses. Sometimes sustained estrogen production will thicken the endometrial lining, with consequent degeneration of endometrium, which alters the amount of bleeding, and can account for alternating episodes of bleeding.