I had 2 week long periods in a 5 week cycle, a week of which were so heavy it really worried me. It turned out I had PCOS but basically because I didn't ovulate the lining just kept building up and up (hyperplasia) until it couldn't anymore.
They did a hysteroscopy to check for fibroids or polys as this may be causing it.
When they did the scan did they measure your lining. Mine was 16mm and it was supposed to be under 8mm I htink.
People don't realise how debilitating long periods can be, you can;t go swimming and have to keep carrying tampons/pads. I hope you get this sorted soon.