If the drain pipe had a hole or crack in it (it usually happens on the bottom but not always), and it got partially plugged, it wouldn't normally get that high, but while plugged could. On a fiberglass shower, it is possible that the floor was not fully supported, and it can flex. May be that when you stand just right, it bends enough to allow water to drain down around the outside of the pipe. If built properly, there should be a vapor barrier behind the tile that leads to the inside lip of the pan that should prevent any liquid water from getting behind the walls and leaking out, so eve a bad caulk joint, if built right, should not allow liquid water outside of the shower.
It could be totally unrelated to the shower, and maybe a leak from the roof that coincidentally happened after taking a shower. Severe shower or melting snow?