It is possible to get eye dilation after head injury if you got brain injury. And there are many other factors that can cause dilated pupils. In details, if you try to focus something in low light condition, you can get eye dilation. Also, drugs, medications and poisons can also be the caused of eye dilation. So, we can't say that you got head injury just only from an eye dilation. You'd better check it by a doctor for sure.