When I lie on my back at night I feel my heart pounding in my chest, and feel pulse in my back of head. Why?

Probably normal. At night, we have less distraction from our environment and are more aware of our body. It is possible that feeling a pulse in the head and awareness of your heartbeat is entirely normal. I recommend that you see your physician for an evaluation to make sure that no other problems are present. It is likely that these sensations are a normal part of the body's functioning.
Because they are. Your heart is supposed to beat in your chest & its pulse wave travels throughout the body. At your age we all start to notice much about our bodies that we never paid attention to before. If you fixate on these sensations and believe they are new you become anxious that something is wrong. If you wonder if something hurts, it doesn't. When something is truly wrong, you'll know. Now it's anxiety.