I don’t believe it!
Definition 1 of 2
Pro Tip 1/3Expression of Surprise
Emphasizing astonishment, usually for positive events or outcomes. 
I don’t believe it! She really pulled off a surprise birthday party for me!
Pro Tip 2/3Not Literal
Often used figuratively, not implying actual disbelief but rather to highlight surprise. 
I don’t believe it! They’ve actually finished building the bridge ahead of schedule.
Pro Tip 3/3Emotional Impact
Conveys a strong emotional response, usually positive excitement or amazement. 
I don’t believe it! My painting won first prize at the art fair!