What Causes A Prolonged Menstrual Period?

in December i was bleeding for 12 days so i went to the hospital because it was big blood clots, well they told me they didnt see that i was preg or had a miscarriage they just said it was a UTI urinary tract infection, well jan and feburary i didnt have my period, now its march and i have had it over a week again and still going, any idea what it might be?