It would be nice to interpret it as "her butt went numb." However, the next line in the song, "Woke her up around 1. She didn't hesitate to call Ice Cube the 'Top Gun,'" clearly indicates that he literally made her fall asleep. This is not necessarily a bad thing; if you have pleasured a woman adequately, she will generally be one of two things when you finish: tired or hungry. In this case, Ice Cube has pleasured his woman well, and so she has fallen asleep, her sexual appetite sated by his prowess as a lover.