Why would I bleed very heavy with big blood clots, change pad every half hou, I had my iron and thyroid checked its fine what could it be?

Need evaluation. Heavy bleeding (menorrhagia) that you describe can be constitutional ie. No physical cause, but could be something organic like fibroids, polyps etc. You need an examination which may need to include ultrasound, sonohysterogram. If nothing is found management would depend on your age and desire for future fertility. Could be controlled with birth control pills, Mirena (levonorgestrel) iud or lysteda, ablation.