I Have Been Having Irregular Bleeding For A Month Now

I have been having irregular bleeding for a month now and I'm also passing large clots.My cycle has been irregular for several months now where I'm having a period every 2-3 months,then when it comes it stays for 2-4 weeks.I have had a pelvis Us and biopsy which both were negative thank the Lord.I feeling very weak and tired.I was given birth control pills to stop the bleeding and in the past it has worked but this time the pills are not helping.I'm not feeling well at all