This phrase simply means that someone is bugging you about what is going on with you. The phrase is a playful way to tell someone to back off of you and to get out of your business.
This phrase is especially useful in the workplace where many people are in close quarters.
ADAM:(from the next cubicle): Wow... that smells good. What is that? Where did you get that from? Can I have a bite?
MARY: Dude, why are you all up in it?
MARY: Hey, was that really Sally that you just got off of the phone with? Are you still going out with her?
ADAM: Damn! All up in it!