My lower abdomen hurts after masturbating for a long time. Should I be worried?

Not usually. Assuming no vigorous penetration is done, lower abdominal soreness may simple result from uterine spasm that may occur with orgasm (clitoral/vaginal). Also, some women with endometriosis may note pain with sex/penetration but not necessarily with clitoral orgasm. Without hore history, it sounds more like uterine "soreness" from orgasmic spasm. Doesnt sound like anything pathological.