To understand if there's anything wrong with your beer, we've got to get more specific than "bad." Common unwanted flavors detected in beer include: cidery, vinegary, fruity, bitter, skunky, musty, buttery, solvent-like, and medicinal (band-aid or plastic-like).
Each of these common off-flavors can be thought of as different symptoms which can help diagnose the problem. Most off flavors are caused by simple sanitation errors. Whether the brew keg was opened during fermentation, or the spoon didn't get a proper soaking in the sanitizer, at some point bacteria was probably introduced into the beer. Other problems are caused by fermentation temperature issues. If the beer is too warm during fermentation, it can lead to fruity, spicy, or medicinal flavors. Too cold, and your beer will be syrupy, sweet, and under-attenuated. Pin-point your beer's specific off-flavor, then find it on the FAQ page for more information.