Seed oils do not have a very long a shelf life, so it depends on how long you've had it. Personally, I keep my sesame oil in the frig; but my sunflower oil on the counter. Even at that, I noticed my last bottle of sesame oil had exactly what you describe in the bottom inch or so, so I threw it out. That bottle had been around for months, so it was probably at the end of it's life anyway. The bottom line is, Yes seed oils can go bad quite quickly.