Can a concussion be dangerous months after you recieved it?

Concussion. Generally injuries related to concussions are self limited to the period immediate to the concussive injury. However all brain injuries tend to be cumulative an isolated "once a done" concussion should not cause symptoms months later.
Concussion. Yes depending on the length of your symptoms can predict future outcomes. If you haven't been cleared properly and return to activities that are prone to concussion can be lethal. Moral of the story is be cleared by a concussion specialist or a physician trained in concussions.
Depends. A common sequela of concussion with actual loss of consciousness is the long term presence of daily, severe head aches and light sensitivity. These result from diffuse brain injury from the concussive force, can last for years, and are a common event in soldiers returning from the middle east after concussive events from ied's.