Rigging character breaks mesh

This is my first time attempting to rig a character in Blender. I’m using Rigify and I’ve followed a full tutorial on how to use it but whenever I try to parent the rig to the model it moves one specific part of the mesh. The part of the mesh has previously been joined to the main body so I don’t see any reason why this should be able to happen. I’ve attached some pictures to show what I mean. Please help I am falling apart

This is what the character looks like before parenting by the way