Crunches and sit-ups will likely make the problem worse since they grow the muscle you are contracting and it's prone to buldge out if not trained to stay tight (pulled in). If you search the net you will likely find lots of articles about crunches being not the shit for the flat belly. What you might need to do is to learn to hold your abd tight and concentrate on the transverse abdominals. Along with losing more fat - which is the principal thing, as abs are made in the kitchen.
The things that will be helpful are:
Core strengthening: serious squats and deadlifts with a barbell
Planks: straight and side and push-ups - done properly they teach your muscle to stay in, and address a women -specific issue of losing the 'V" shape when thy start lifting
for transverse abs, Turkish get up with a KB and kneeling cross-body chops and lifts on the cable machine will work.
Kneeling on all fours, than expelling forcefully all air and holding it like that for 15-30 sec, for about 10 reps will help with contracting the muscle (Cat Vomits).
from the crunches family, you can try the crunches that support your back on the BOSU and make you extend and lean back, rather than crouch into a full sit-up or knees to elbows crunch.
Hope this helps