There's a fine line between real pain... and "hard to do."
Ex: DB Military presses not being able to push the weight up anymore (hard to do).
Someone else doing the same exercise, ect... has pain in their rotator cuff. (PAIN).
Don't do it if you feel pain, that's how one becomes injured.
Also, if you are strengthining your abs... you should also do back hyperextensions to tighten up your lower back muscles because the newly worked abdominal muscles will pull on your lower back.... it's all about balance.