What actually occurs inside the brain after you get a concussion?

Not clear. The actual changes in the brain after a concussion are not properly understood. After concussion, post-concussive syndrome (pcs) can be from mild to severe. It can include a range of symptoms including (but not limited to) headaches, dizziness, nausea, poor concentration, irritability, anxiety, noise or light sensitivity, depression, fatigue, sleep problems, memory problems.
A lot. During a concussion, the brain actually suffers injury from impact and from bouncing around in the skull. Essentially it gets bruised and makes you more susceptible to further concussions if you return to sports or activity too soon. There are a myriad of obvious and also subtle concussion symptoms and the patient should be evaluated and followed by an md until resolution.