Think about it: it's repetitive motion of a batch of muscles that aren't the most well-used in the entire body (I know my own abs are really out of shape). The average person's abdominals, trapezoids and obliques aren't used anywhere near as much during day-to-day living as, say, the biceps, triceps, calf muscles, glutoids and deltoids.
Vigorous sexual activity, on the other hand, involves repetitively straining those muscles against a heavy load (either your own bodyweight, or your partner's, depending on position.) Try doing that many reps against, say, thirty kilos at the gym - you'll fatigue quickly if you're not in shape.
For somebody like Faye, who really doesn't get enough regular exercise, and probably hasn't focused on those particular muscle groups when she does, it's not exactly surprising if she pulled a couple of muscles.