they probably couldn't get a good view of it. If it's too low, they can't get the wand in a position to correctly measure everything. It's nothing to worry about.
Like bridget said, if it's the placenta, it's a little more to worry about. Because if it's covering your cervix, you will HAVE TO have a c-section, or else there's a HIGH chance you'll bleed to death
Good Luck!