Mesh A changes after joining it to mesh B so im tring to make my first character and everything was fine until i made the hands and when im done with it i tried to join it to the rest of the body (Ctrl+j) but whenever i do it be smoother/changes the color or whatever you want to call it 
(i applied all of the modifiers before so there are none)

so i got 2 questions

1. how can i fix it?

2. why it changes\what is going on?

Before

After
I too dont understand why the smoothing is happening. But you can fix it by clicking in the flat option, and flat shading should be applied to the whole object again. Maybe someone else can explain what is happening.