Can a co-signer take a car back? I have never missed, or been late on a payment. She wants to sell it.?

I had someone co-sign on a car for me two yrs ago and she is threatening to take the car and sell it. I have NEVER missed, or been late on a payment and obviously need my car. Can she legally take the car away from me? I cannot refinance it because of my credit. (I owe about 15K in medical collections).