16 day period

I came on my period the day before I was going to a 4 day festival so I just figured I'd deal with it because that's what girls are taught to do. But then it just got heavier and heavier and there was no end in sight... I was going on holiday the day after the festival but I figured oh I'll be fine my periods last a week maximum! But now after my whole holiday of bleeding I'm at home on day 16 of constant bleeding with massive blood clots, I feel so weak and helpless i'm losing a lot of blood and I just feel like a pain going to the doctors over excess bleeding considering I've had every blood test under the sun to check what's up :/ anyone any ideas on why my body isn't working properly? Much appreciated