Why am I skinny, but have a big belly?Watch
Another thing people tend to overlook is sauces - the ones you buy are typically full of sugar - wether that be ketchup or something like dolmio /ragu etc. It's better to make your own.
It could also be that you just haven't been going to the gym for long enough to lose all the belly fat/flab yet if you are like me that will be the last place the fat/flab leaves and it takes time.
You are skinny and have a low % of muscle. Bulk, make gains and it'll all go away
Ab exercises don't specifically remove fat from your stomach, NO EXERCISE DOES. You lose fat by expending more calories than you consume. This means that you either do more exercise or eat less calories. There is no way to specifically remove fat from your left leg but not the right leg for example, unless you have surgery. Fat can accumulate all over the body, some people have more in different locations which gives birth to 'body shapes' and saying people look like ''pears' or 'apples', which would explain your slim arms but big belly.
There could be plenty of reasons you are not seeing your stomach drop;
1) Are you being true with yourself, are you eating less calories or doing more exercise CONSISTENTLY? Exercise does not burn as many calories as you think
2) Do you eat junk food? You could be seeing visceral fats (organ fat) which would require a healthier diet and more exercise to get rid of
3) Do you have any underlying medical conditions such as IBS or allergic reactions to food which inflame and bloat your stomach.
For future reference, a 6 pack is only visible under low body fat percentages. Search for body fat comparison images to see examples of this, i think 8-10% is what is required for men to see their abs.
Hope i helped.
If you work out now you should gain quite nicely on your arms and then go for a cut by February and you could have a great body ready for next summer.