It sounds to me like you weren't grounded enough in the dream itself. A lot of times when first becoming lucid in a dream, you need to ground all of your senses to fully immerse yourself into the dream scape. The best way to do this is by focusing on each sense, one at a time, and utilizing it in the dream.

In your case, it was your dream body that had not fully transitioned, as dolphin suggested above. Do you recall if you actually looked down at yourself? (hands, feet, legs, etc.) If you happen to get stuck in the grounding process, good old verbal commands usually work wonders. For example, saying something like "decrease gravity" may result in decreased gravity, and increase the ease of motion. You may also want to start small, moving your fingers first, then forearm, then the shoulder, etc.

To answer your other question, yes and no. It is very possible to become lucid, and then lose lucidity and fall into a normal, unconscious dream. If at any point you find yourself losing lucidity (whether it be the dream coming to an end, or because you are fading), ground yourself again. Dream spinning is a great technique used to both stabilize a dream that is collapsing, and to transition into another lucid.