If the pressure tank fills and then empties fairly fast and then it cycles over and over, the check valve in your submersible pump may have failed. Or you might have a leak or leaks in the pipe down to the pump. You can check for leaks in the rest of your system by shutting the main valve off.
After having the internal check valve in one of my submersible pumps, I now always add an external check valve just above the pump. Of course it requires pulling the pump to install the additional check valve. I suppose another way of dealing with the problem would be to add the check valve to the line to the pressure tank, but I think the water would still drain back down the pipe quite a ways, leaving a vacuum in the upper area. Not so good.
Every well and every system is different. Hope you can find out what is going on with yours. Good luck!