Limiting the number of medical providers was part of an effort by insurers to hold down premiums. But confusion over the new plans has led to unforeseen medical bills for some patients and prompted a state investigation.

More complaints are surfacing as patients start to use their new coverage bought through Covered California, the state’s health insurance exchange.

“I thought I had done everything right, and it’s been awful,” said Jean Buchanan, 56. The Fullerton resident found herself stuck with an $8,000 bill for cancer treatment after receiving conflicting information on whether it was covered.

“How am I going to come up with that much money?”