This used to happen to me all the time.

I don't think it's sleep paralysis in a dream to be honest, but rather a lack of experience.

When I first started LDing all my LD's were like that. Everything I could do was completely restricted. If I could fly it would only to a height of a few hundred feet. I remember one dream where I could 'fly' but it wasn't really flying. It was like jumping really high since I couldn't make it last.

It's this type of lack of control that occurs when you either are new to lucid dreaming or just haven't had one in while. I noticed clarity and control greatly improved the more regularily I became lucid.

Now that I haven't had one in so long, my one last night was pretty hard for me to control. Because of this I think it's just experience/how used to being lucid you are at that time. But hey, I'm only guessing based on what happens to me and you could be completely different.