Some rigs are plumbed to fill the FW tank from the city water connection. There will be a clearly marked valve in the wet bay for such, AND there won't be a gravity fill. If you have a gravity fill, you won't be plumbed to fill by the city inlet.
Let stated above, in most cases like these, there might just be a little dirt in the check value, or possibly the check valve was forced open. Disconnecting the city line and running the pump for a little bit often can reset it.
There's no use crying over spilt milk... unless it's on your keyboard.