either becuase of, buying too much stuff, debt interest, ship upkeep fees, or infrastructure maintenince
chances are you forgot to disband your fleet, and if you wish to keep ships, they cost money per turn to maintain, called upkeep. this is what may be lowering your funds, but if you go into debt, there is debt interest, which, at a certain point, will increase to a value of which you will not be able to escape. and you will have to restart.