A photo would confim, but it sounds exactly like an infertile egg.
Females can pop these out any time, once they start ovulating. Most of the time, unfertilised egg follicales are reabsorbed (to save physical resources), but sometimes they're passed as infertile eggs.
Sometimes you get whole or part clutches that are infertile, sometimes just the odd one or two a few days before a clutch of fertile eggs. Sometimes you just get odd singles like this.
It doesn't mean that mating has been unsuccessful. As long as she's OK, just carry on as though you're expecting her to lay a clutch of fertile eggs.