Felt Strong Fear in my Dream

I was trying to sleep and ended up falling asleep without knowing it. Well in the dream I was still awake in my bed, trying to fall asleep so I went to my computer. The room was dark and I looked over and sensed there was a malevolent entity in the room. I felt the adrenaline rush of a chill that runs down your neck and into your spine when you are terrified and it was realistic. I also felt like I was gonna pass out and started to fall from my chair. This is the point where I woke up.

Also, a few weeks ago I had a nightmare so terrible I began screaming in my dream and woke my parents up. They came running in to see if I was ok and I couldn't remember a damn thing about the dream.

So these are the questions I have:
How could a feeling in a dream seem so real?
Can a dream be so terrifying that when you wake, your mind blocks it completely out?

Why do feelings "feel real"? hmmm I don't know that either, so why can't feelings in dreams "feel real"?

I haven't had a nightmare that scared that much in over 13 years because I think that there are no such thing as good or bad dreams there are just the perception of them, when my dreams start to go wry i just start trying to find good things in them and the bad feelign dissapear