What can I do? abdominal workout not working! UGH!!?

I am 5″7 and on any given day, I range from 117 lbs to 121 lbs.
I admit, I am skinny. My arms and legs are sticks but I have some stubborn lower tummy tub. It’s not very much at all, just enough to annoy the hell out me from time to time.
I workout EVERY morning with stretches then crunches, reverse sit ups and then the bicycle workout (crunches where you bring your left elbow to your right knee and visa versa) doing each set 100 times. my upper tummy is gaining muscle and is (for the most part) flat. my lower tummy…nothing. I do feel muscle behind the fat but the fat wont go away. What should I do? How do I get rid of it? Help!