What happens when u have negative cap space?

The AI will cut players to get you under the cap space, starting with the higest benefit for cutting (if you switch your roster view to "contracts" it will be the higest "save if release" players). If that brings you below the minimum 45 player roster, the AI will continue cutting until you have enough cap space to sign up to the 45-player minimum with base salaries, and then will fill your roster to 45 players.

This doesn't happen except on a game stage, so if you are in negative cap during the free agency stages you have until the end of early free agency to get it sorted out.