Why are there spots of pee all over the carpet?

I have an 8 month old, neutered, male Rottweiler mix. He has been house trained, and has been doing well for 2-3 months. He recently started barking and growling at noises, us getting home, single people approaching after dusk (even if he's met them before), etc. He also has peed in the house the last two days, something we trained him on and has been doing so well with. I walk around barefoot and have also been noticing wet spots on the carpet in every room in the house, no bigger than if I filled my water glass a little too full and carried it to the bedroom. Upon closer examination and some paper towel blots I have discovered it is indeed pee. Ive read the similar postings, but would like as much advice/info anyone can give after reading my specific situation. Is it just a puppy phase? Should I be worried? How can I fix it? Could it be serious? Thanks to all who have something to offer!