My little one is now missing her tail.

Hi, Guys. I have two northern diamondback terrapins. One is about 4 inches (shell), and the other one is about 2 inches(shell). They are in the same tank and have been getting along so far. Yesterday, my wife noticed that the little one does not have her tail anymore, all she has now is a base stump. I don’t know what and how it happened. I don’t think terrapins shed their tails. Can someone suggest somthing. Should I use any medication to help her heal? Any help is greatly appreciated. Thanks. Alex.