My last few periods have been so painful and weird

Hi, I'm 15 and I have had a really weird period. I have had my period since I was 11 basically 12. My periods have always lasted for 1 day not even that. I get it at 10.30am on a wednesday by that night i have spotting and then by next morning nothing at all. in the time I actually have my period it usually doesn't even fill a pad... I may use maybe 2 due to having to change them every 4 hours for hygiene purposes. My cycle is usually 35 days plus. Anyway I have been having really bad abdominal pains and have spent 11 days in hospital. The doctors have no idea whats wrong but I think its to do with my period. My last few periods have been so painful and weird. The pain is unbearable. Painkillers do nothing. the weird part about my bleeding is that I'm not getting ANY bleeding at all unless i go to the toilet and when I go to the toilet its just massive clots but like (pre warning and apologies for how this may sound disgisting) but when I'm on the toilet this thick clot like mixture just strings down and hangs there and I actually physically have to break it off with toilet paper otherwise it just keeps hanging there. WHAT IS WRONG WITH ME? PLEASE HELP!!!!