What happens when you get hit in the sternum with a lacrosse ball? should i be worried about pain?

Some basics: The sternum is a bony structure that sits atop an inflated lung that will cushion any blow. It can be bruised or cracked. It's points of movement with ribs can become irritated & make breathing painful. Ibuprofen & a cold compress can reduce pain. Over time it would heal like most other body structures. A crack would be hard to document even with x-rays, but would not change treatment.

Nothing much: Over time it would heal like most other body structures.