Sleep Paralysis Or Something Else?

Note: I'm new to this sight and for months I've been trying to figure out what happened to me this one night, and I'm hoping that some input from people on here will help.

My fiancé' and I used to live in an apartment in Vancouver, WA, while I was pregnant with our son, and a lot of weird things would happen there but by far the strangest and most terrifying was the night I woke up unable to move and screaming.

The night really did start off like any other night-we got into bed, talked some, he rubbed my belly and said good night to the baby (I was about 6 or 7 months along at this time), and then we drifted off to sleep. But things got scary a few (what I'm guessing) hours later.

I woke up screaming and feeling like I was being held down. I couldn't move at all save for my head. At first I thought I was dreaming, but this was no dream. I was in my room, sleeping next to my fiancé with my dog at my feet and the light from the neighbour's porch shining through our blinds, just like usual. My surroundings were very real and I was genuinely terrified. I remember screaming and screaming for Matt to wake up and to help me but he and the dog just stayed asleep. I tried to struggle, but the more I struggled and screamed the more scared I became. I felt a fear that I had never felt once before in my life, and the feelings of loneliness and hopelessness that consumed me were unlike any I'd ever felt. I didn't know what was going on, but after what I, again, am guessing, was a few minutes, the feeling lifted and I was able to move again. I remember lying there for a few minutes after, trying to collect myself, and eventually I did fall back asleep but since then I haven't forgotten that night, and I've been trying to unravel its mystery.

I swear I can remember seeing black shadow-like figures standing next to my bedside but the room WAS dark, so I'm not sure. This has never happened again but I would still very much like to know what happened that night.