If your mother's insurance company was aware of the situation, your mother insuring a car that has the loan under your name, and there were no exclusions in her policy that would preclude you from driving the car and being covered under her policy, then the accident should be covered under her physical damage coverage.

Your mother speak to her insurance company's agent and place the claim to find out for certain if her policy will cover this damage that was sustained in the parking lot while you were out of the vehicle. If for some reason it is not covered then you would need to see what state laws say about liability in a case like this. A lawyer or your state's regulatory body should be able to help you determine your responsibility if the insurance company denies the claim.