Why am I getting billed for services my insurance covers?

I have Medicare (part A and B) and Medi-Cal. I get Supplementary Security Income because I've never been able to support myself. I've never been married or co-habitated with anyone after I left home at age 18. I don't have any savings, bonds, property or anything else of value and I've never received gifts or financial help outside of my SSI checks so I am fully covered by the insurance. However I continue to get bills from collection agencies for services that Medicare and Medi-Cal cover. They are billing me thousands of dollars for emergency room visits, CT scans, blood tests, ambulance rides. I can't afford it. Why dose this happen and how do you deal with it?