I'd suggest a visit to the vet first. Or at least make a phone call and discuss it with them. It could be medically related.
If not, I'd go back to basics. Go through the same steps you took in the beginning: Limit freedom around the house, take her to the pad every few hours and ask her to pee. Reward successes, don't freak out over mistakes. The fact that she's sneaking it means she knows it's wrong and doesn't want to get in trouble.
Here's one Web site that discusses adult dogs have setbacks. Hope it helps:
How to House-train Your Adult Dog