Always Reconfigure Your Rig

If you make any changes to your rig, move a bone a little. Delete a bone, rename a bone. 

Reconfigure your rig in Unity.

Not doing so can create a myriad of problems, such as bones becoming long and contorted, or turning in odd ways.

Go to your model file, go to "Rig" in the Inspector window, then click "Configure" once you've done so, scroll down until you see the "Pose" button, click that and click "Reset Pose" then after that, go into Pose and click "Enforce T-Pose" then hit Apply. This should automatically fix any issues with the model. Note that if you have any animations that affect non-humanoid bones then you may have to fix them, this is just part of the process.