never ending period, wtf

10 or 11 days ago my period started. The next day it stopped completely and returned 24 hours later. It was normal, but continued longer than usual, stopped here and there BUT NOW ON THE 11th day I am bleeding so heavily, its clotting, the past 24hrs has been bright red, lots of running blood and large dark clumps. I've never experienced a period this long or a flow this heavy, and non stop. And to top it off I was on a date last night had to change my tampon 4 times and today at the beach it was worse, and am starting to get very worried, feeling light headed and unwell. Hit me possibilities of what it could be and if you've had anything similar happen?