Self filling fresh water tank on City Water

OK group -- have a little problem that I havent experienced before - either in my drivway - or long stays at campgrounds hooked up to the campground water -- I returned from dinner tonite and found a puddle under my fresh water tank filler cap. i looked under the bunk and found the floor dry at the tank and the hot water heater -- however i found that my fresh water tank had filled itself !!! ond is now overflowing -- not with a tremendous stream but just a slow trickle -- I have a shut off valve from the tank to the water pump and that was put in to make winterizing easier (have a hose to draw the antifreeze from the bottle - so i shut off the tank) - I'm baffeled - this has never happened before in all the trips i have done since last year -- the water pump still works - Thoughts I'm thinking a check valve in the pump wore out??? - so as a fix -- I jut have to remember to open the tank when not on city /camp water