Blendshapes manipulating Skin Weights On Rigged Character

[d_maktabi1]

Hi all, I've been trying to learn the manual rigging process for my character, following every step as recommended by pros. I first created blend shapes on a separate mesh of the head of my character, then I combined and merged the head back to the body and rigged the body, skinned the rig and everything works ok individually, but when I using the rig and blend shapes at the same time, changing the blend shape values makes the skin weight on the body go crazy and effecting somehow random parts of the body as you can see in the video I uploaded. 
I ask you please explain why this happens and what part of the process I did wrong? is there any way to fix this

situation? 

 Thank you!

[mcw0]

From what I can see in the channelBox, it looks like it should work.  Is the origin of the blendShape set to "local"?  Make sure it is.  That's the only thing I can think of without seeing your graph.