Is it normal my coffee tastes old/burnt

I am not that used to making black coffee (usually a cappuccino drinker) but I like black coffee from time to time but I feel like even though I prepare it pretty much the same each time and always comes out tasting differently ? How is that possible ? At first it tastes good then after two weeks or so it starts to taste horrible. It tastes like when you have left your coffee on warmup for hours and then grab a cup and its gone bad, or like it has been burnt badly. Ive tried many different brands of coffee among the usual popular ones and it makes no difference which kind I buy. I fill up enough water for two cups of coffee and I take two scopes of coffee, sometimes slightly more. Am I using too much coffee and too little water ? I read somewhere that you take one scoup per cup and sometimes that turns out tasting amazing but then after a while it no longer tastes good. Does coffee just go bad really quickly even sealed ??? Help please