Irregular l bleeding/Hemorrhaging?

I have been bleeding for about 8 months straight now and here lately I am going thru like 1 tampon an hour and I have to wear a pad on top of that. I am getting really concerned, I have gone to the doctor who put me on birth control pills and I have had my thyroid checked and everything is great. I had a sonogram outside and inside and they can’t see anything. Has anyone had this problem? I am so stressed, I mean how much blood can I actually continue to loose with out dying.