On for 16 days then 8 days later I'm on again and more painful and heavier.

Are you using any form of birth control?

If not, it might be the time to try a pill (patch, ring, or whatever you find convenient and comfortable).

They're known to lighten periods, relieve menstrual symptoms, and give you control over your cycle.

It is always best to talk to your doctor about any changes in your menstruation. Taking advice from someone online can be dangerous, as we can't give you a diagnosis online.