Wake up shocked

On some nights I will wake up from dreaming with a shock, I will be in a dream normally a normal dream, not a nightmare or bad dream. Normally something will happen in the room, such as a pain about to happen. For example last night I was On a road and turned around and I got shot. But instead of taking the bullet I woke up, and shot up as fast as I can react, wide awake and my heart feeling like it missed a beat.

It normally happens if I take a nap in the afternoon, but sometimes I move when I wake up. Another example is I was walking down a street, and went to cross a road. I stepped out and a bus hit me in the dream. But the wierd part Is that I was in the middle of a king size bed, not near any Walls or anything I could have pushed off, I managed to fall out of the bed and land on the floor. Even wierder is that where the bus had hit me in the dream hurt when I woke up, even though I landed on the opposite side. Does anyone have any idea why this happens to me?