Alrighty Nymphomania flaw, here we go, bear with me.Nymphomania(Flaw)
: Certain parts of the body are more sensitive than normal, usually breasts(for women) and genitalia that are affected. This causes the individual to become aroused rather easily depending on the body part and level of increased sensitivity. Along with increased sensitivity are intense periodic cravings of certain forms of stimulation, usually a form of stimulation the individual hasn't experienced in awhile.
For men this problem occurs about once a month, for women it tends to take affect right after their normal monthly cycle. The effects between both genders are the same however, depending on the type of stimulation, certain parts of the body will begin to tingle and itch differently. It will be very minor at first, eventually growing into a full on aching need after several days. This is the first clue to what their body is craving, the second is that soon after their period starts(even the first night), they're treated to increasingly vivid and intense erotic dreams. Once their nymphomaniac period begins, each masturbatory orgasm becomes less and less satisfying until they find that they can no longer bring themselves to a climax with self stimulation. After a couple of days this problem starts affecting other forums of stimulation until the craving is fully satisfied.
The intensity of the craving and resulting effects gets worse and worse with each passing day until the craving is satisfied. While the first day may not be all that different aside from being constantly horny, the fifth day has their bodies in a never ending state of full arousal.
So that's pretty much the fluff. as for mechanics I'm not sure how exactly to handle it. Thinking a table for the type of stimulation craved(which would go along with the character's kinks and limits, for example; if the character did not like anal, anal stimulation wouldn't be a possibility). As well as a cumulative penalty/bonus for each day that passes without having the craving satisfied. Penalties would likely affect most skills(especially those that require concentration) due to the distraction of intense arousal and desire. This would also affect spell casting, and maybe even attack rolls. However, bonuses would likely be given to diplomacy(only when seducing) and to Perform: Sexual Techniques. Thinking that the penalty would be increased by one for each day that passes. First day(start of the period) would be -1/+1, the fifth day being -5/+5, and so on.
Hope that is not confusing & rambly, feedback is welcomed!