Hello! Welcome to the puppy stage- yes, you are likely experiencing regression- it may happen once, twice, or a million times before your little one is totally potty trained. At 15 weeks old your little one has very little control over his bladder still. Maybe he needs a smaller area to be contained while you are gone- is your kitchen large? Until mine were about 6 months old, they only had the space of a single expen while I was gone- just enough room for a bed and water on one side and a potty pad on the other.
Dogs LOVE to chew up and drag around pee pads
I suggest either getting a pee pad tray like this
ClearQuest Puppy Pad Holder at Bizrate - Holiday Gift Shopping 2010
or switch to washable pads which aren't easily destructible- you can buy them from either of these places- both YT members with great products and even better service.
Puppy Pads - Puppy Training Pads - Puppy Pee Pads Washable and reusable dog and puppy pee pads, belly bands, and diapers at T. Bumpkins & Co.
When you are home- you need to have your eyes on the puppy at all times, if you can't he goes into the ex pen. While you are watching him, take him to his pad every 30 minutes or whenever you see signs he may have to go and tell him to go potty- if he does, give him a treat and lots of praise!
Potty training can be a struggle and usually takes months to acheive- good luck! it does get better.