What Causes Heavy Periods With Mucus Mixed With Blood?

For the past year I have been getting really heavy periods with large liver looking bits in it. Last months period was nothing like that, it started as mucus mixed with blood and had one day of what I would call a normal period the rest of the days again mucus with blood. This month on my due date I got the same thing except its been mucus and blood only no normal period. My usual cycle is 28days. What is happening to me?