Don't workout immediately after a meal. Give yourself some time to digest the food. You feel weak cause all the blood is pumping in to your stomach to digest all the things you've eaten.

If crunches feel painful, which it shouldn't cause the aren't the hardest core exercise, try lying knee lifts instead. To do a lying knee lift, lie on your back with your knee up at a 45 degree angle then lift it up you legs towards your stomach then bring it down however don't let your feet touch the floor. Keep them at least 2 inches above the floor while still maintaining 45 degree angle before lifting back up again.

The lying knee lift will not only work your core but also your glutes and is better than the basic crunch.