“I knew it!”
Your friend comes over to your house for a party but doesn't drink anything. You wonder if she might be pregnant. A few weeks later, she tells you that she is. You say this in excitement.
I knew it!
This is something that you say when you suspected or guessed something, and then find out that you were correct. For example, when you thought that you were going to fail a test, and it comes back with a failing grade, you can say to yourself:
Crap! I knew it!
You can say "I knew it!" about either positive or negative situations.
It's pronounced with a strong stress on the word "knew".