Some times when I wake up, my heart is racing and I'm breathing heavily. I noticed that this usually happens when I wake up too early to have made it to REM, like I was in slow-wave sleep. Since that's the part of sleep that night terrors happen in, I thought that might be the case. Still, usually night terrors don't come from nowhere, they happen if you had surgery as a 3 year old and other traumatic events so i'm not sure.

Sometimes I'm not even sure that I fell asleep at all because it happens pretty quickly, I don't know if this applies to you... but there's my 2¢