In some states, people with SSI are categorically eligible for Medicaid.
In those states, the income limits for Medicaid for the aged, blind and disabled are quite low. As a result, when or if a person is switched from SSI to SSDI, they lose their Medicaid eligibility, due to their income increasing.
As Mercy said however, many states have programs whereby disabled or elderly people can qualify to have their Medicare Part B premiums picked up by the state. It sounds like that is what you are receiving.