House Trained Dog Peeing Inside?

My 2 year old, 14 lb. terrier mix is intermittently peeing in the house. I've had her since she was 4 months, she had stitches from spaying. I umbilical trained her with the help of a trainer. I would shut her in my bedroom with a pee pad. She has a dog door too.

It's random, it used to be at night, but I never see her do it. We're trying, not the first vet or first time, antibiotics and prion in case she has contenancy issues. She's been on the drugs since Wednesday night and tonight when we got back from the dog park I hadn't given her the meds. I was in the kitchen and she came around the corner and peed on the floor by the vacuum. We already went the route of possible crystals. I have an irregular schedule, it can't be helped.