The state Department of Human Services has warned health insurance companies that it may not make payments for Quest — the state’s health plan for low-income families — for the last quarter of the fiscal year, leaving insurers to absorb about $300 million in medical expenses until at least July. The potential delay in payments has stunned insurers and alarmed health care providers, who worry a delay could jeopardize the ability of insurers to cover claims, which would cause cash-flow problems and influence how some providers care for Quest members. “That would be extremely frightening, because if the state isn’t paying the plans, then the plans are not going to be paying all of the people who actually deliver the health care services,” said Beth Giesting, the chief executive officer of the Hawai’i Primary Care Association, a nonprofit that represents community health centers. “Of course, that provides a ripple all the way down.