Dog has started urinating in the house
We have two lab/pitbull mix sisters - both came from an animal rescue shelter; both are 16 months old. They are just over 50 lbs, very active, and generally well behaved. Recently one of our girls has started urinating inside the house. She will leak drops of urine, sometimes a brownish color, and then just go. (The dogs have been house trained since they were 4 months old so this is a new development.) The sister who is urinating in the house is the low dog on the totem pole. Her dominant sister rules her but follows our commands very well. Neither dog acts sick, both run and play as often as they can, but the dog that urinates does it without warning. Could this be a behavior problem or perhaps a urinary tract infection?
PS We switched them to adult dog food when they were a year old and the dog with the problem became a picky eater - as opposed to gulping down her food like she used to.
Any advice is greatly appreciated.