What’s the difference between
I didn’t expect that
And
I wasn’t expecting that
And what situations can I use them

Question

What’s the difference between 
I didn’t expect that 
And 
I wasn’t expecting that 
And what situations can I use them

Answer

I didn’t expect that is the past simple (used for a short actions in the past).
I wasn’t expecting that is the past continuous (used for a longer actions in the past or habitual things in the past).

Hope this helps;)