Well, seems like you've experience sleep paralysis. It happened to me quite a few times. The feeling of being locked down can surely be frightening.
I was once living with a roomate. We just moved in there. I had 3 sleep paralysis experiences in two weeks there. But, before that, I only had sleep paralysis once in my life. I guess sleeping in a different bed made this happen, plus I never sleep on my back, but when I was there, I managed to fall asleep on my back somehow.
Each of those times I tried to move and scream, but I couldn't. It felt as if something was holding me back. Visual hallucinations happen once you're in this state, becuase your body and parts of your brain is asleep, but you're still conscious. Thus you basically dream while your eyes are open.
Worst of all, you can sometimes feel pain in this state if the visual hallucination is doing something to you. For example, I had one experience where some sort of creature was biting my leg and I actually felt a bit of pain. But I couldn't do anything at all, cuz I was paralysed.
Sleep paralysis also has a higher chance of happening when you're waking up like you do. I mean, you wake up in the morning when your partner comes back and then you go to sleep again. In the morning, we go through what's called REM sleeping, which is the time when we dream.
When you wake up and go to sleep again, you immediately enter the REM cycle, thus, if your body falls asleep, but you stay conscious, you can experience hallucinations.
It is a very curious topic indeed. I have experienced sleep paralysis many times throughout my life but was always a lot more unpleasant. Somehow, this felt different.... but no less fascinating :)