Head of penis squeezing through opening

Scary moment this morning. Woke up after sleeping with the cage on, and the head of my penis had squeezed through the opening, almost thought it was stuck, but was able to get it off. It fits perfectly and I am able to wear it all day, just that morning wood seemed to have made it squeeze through? Any idea why this happened and how to prevent it? Thanks ( I can provide more information if needed to clear up anything)