Pain in the muscles does not mean to stop the exercises, but actually here you can define this pain in two different ways.

One is the muscular pain and the other one is the pain when your muscles feel fatigued which is a good sign meaning that you are doing your exercise in a correct posture. As far as abdominal pain is concerned, if you are doing it for a long time, your internal abdominal muscles get hard and the top layer of fat remains there as by doing sit-ups the upper layer remains intact but your internal muscles develop, so by doing it, your internal muscles get hard and pump quickly resulting in quick failures and when you try to push that a little bit extra that results in your lower abdominal pain which is not to worry about. This may be due to your trying to lift your back upwards while trying to push it hard inside towards your spine.