Are you completely sure you weren't able to move or was it just hard to and you were thinking that it might be impossible?
If it's the first case then like gab and Xanous said, it might be either rare episode of SP or false awakening dream about having SP, SP can happen while falling asleep after all.
If it's the second case then it might have been just hypnagogic imagery, i personally can slip into hypnagogic state very quickly and easily when i lie/sit still, sometimes even too quickly, and in hypnagogic state we are often relaxed to the point of feeling that it's hard/impossible to move but it's actually not.