Aftershock (sexual)

Aftershocks are involuntary muscle contractions which occur after orgasm. The name is derived from the aftershocks which follow an earthquake. It is not clear whether aftershocks are part of the conclusion of an orgasm or are themselves a form of multiple orgasms.

Female aftershocks
Aftershocks in human females last for approximately one minute after an intense orgasm. Aftershocks are usually pleasurable, but can be very painful in some cases where they are similar in intensity to cramps. However, after the pain subsides the overall experience may be considered pleasurable.

Aftershocks have been repeatedly observed in women who do not have voluntary control of the pubococcygeus muscle, which supports the view that they are involuntary. Continued stimulation of the same intensity may suppress or mask aftershocks. The aftershocks are felt by the woman and can be felt by her sexual partner, either around a penis, finger, or hand inside the vagina or through the mons pubis.

Male aftershocks
A Yahoo! Sexual Health article mentions small orgasms that occur after a main orgasm and lists them as one of three forms of multiple orgasm in men.