Motor cycle accident with no other parties involved

My husband was in a motor cycle accident where he lost control. We filed all of his medical bills with our health insurance provider. We also had $2500 of PIP which covered his missed days of work and some of the medical bills that the health insurance did not pay. Our health insurance provider has now sent us a bill for over $5,000 that they want us to pay to them. They are stating that they are not responsible for all of the claims that they paid. Is this legite?