It is possible there is another factor, but first you need to balance out your potassium vs sodium intake. It sounds like your eating habbits include a lot of sodium which will cause muscle fatigue and cramping. I would reccommend reducing sodium intake (fast food, sweets, salt) and start adding potassium ie. bananas, yams, cantalope, papaya, guava, no salt (salt substitute), cauliflower, cabbage.
you're fat... once you kill those nasty eating habits and start eating healthy you'll lose that unattractive 50lbs and get that sexy beach body you want. do other exercises than sit ups. try jogging or dancing...