I agree with the poster who said to watch her feeding schedule. Feed them at certain times each day, and leave the food down for 10-15 minutes. Whether they eat or not, pick the food up after 15 minutes. They will soon learn to eat the food when it's down for them, and therefore their pooping pattern will be on a schedule. Don't give too many treats during the day, as this will change their schedule.
Mine eat at 5 am and and 5:30 pm. They poop around 5:30 am and 6 pm-sometimes with an extra somewhere in between those times
But, at least I know when to expect it!
Also, you might try a playpen with a pee pad in it for the days while you are gone. It's possible that she just can't hold it that long when she does need to go. At least then she could relieve herself while you were out. This should work-ours both will potty inside on pads and outside. We just trained them to do both.
If it doesn't improve soon, I'd talk with your vet.