Benign ovarian cyst? other than that... i don't know. Cysts can come up quickly, hurt and are noticeable at about that size. Caffeine makes them worse, B-complex vitamins make them better. I'd check with a doctor though... ok? use a pen and draw a circle around it, because that way you'll have an objective method to see if it gets bigger, which is an important diagnostic fact.
hope you feel better.