What do my symptoms mean.... and what do i do?

i have been on my period for a year. i am 21 and i have had problems with my period since the first time i had one at age 11. i get large blood clots sometime the size of my palm. ive ben to three different doctor that wont do any tests they just put me on birth control. i am always tired and i am already anemic. cervical cancer runs in my family but doctors wont test me. i dont know what to do. sometimes i bleed so much it cause me to feel sick to my stomach. what could it be, what should i do?