Change in the household would be my first guess, but you say you ruled that out. I would just ask you consider anything and everything such as a change in work/feeding schedule, new person in the house, new food, new neighbors, new colonge/purfume, new plants in the backyard. Dogs key onto the smallest things. Could be a new deliver schedule from UPS, FedEx, or USPS that’s happening when your gone. If you rule that out, I’d take her to the vet, especially since it’s multiple loose stools.
Though you mention the fireworks. If they’ve been especially obnoxious this year in your area, that could be it. She’s a little older now and she’s just keying in on it more for whatever reason.