Misty: First, be glad you have that sump pump. Think of the problems you would be having if you didn't.
What you describe does make you wonder. However, if there is no surface water running across your basement into the sump pump, then the water has to be coming from a underground source and not your hot water heater or what ever. As you know, there is water down "there" somewhere. At any rate, it could be that this underground water source has found the easiest and best route is to your basement. Makes sence because you have dug this big hole in the ground.
You say that it hasn't rained for a while, so this tends to leave out any run off coming from your house, or water running "towards" your house. If your gutters and downspouts and the site in general drain 'away' from your house, then we are back to to a underground water source.
My only suggestion would be to keep your sump pump working properly. I would even have a 'spare' pump hanging around in case there is a problem. You then could just put a hose to the spare, set it in the water filled basement, and take the water out of your basement while you get the other pump fixed. Can't tell you how many times I have had to do this.......and it is aggrevating.
Talk to neighbors. They may know the course the water takes in your area. Talk to the plulmber who worked on your house. He may be able to shed some light. You could also talk to a city or county engineer. Tell him if he knows how the undergound water courses in your area. If he does not know, he may be able to lead you to someone who does. The basement contractor could also have information.