potassium. when i diet i know my intake of potassium is very low, especially when im using dnp and i try to stay away from fats and sugars and pretty much everything that isn't straight up protein. avocados have about the highest daily % of potassium out of any food...if you aren't dieting and can afford them, eating like half of one a day will be more than what most people get in potassium. banana's i normally eat, but the content is pretty low, i cut them out when im dieting, so i have even less.
make sure you are drinking electrolytes, they contain (minimal) potassium. i would say you could supplement and see if it helps, i've never tried potassium vitamins.
also dont just jump in and start doing some abs. get yourself a foam roller, stretch and shit before you just jump right into crunches etc. easing into it will help, and if you start continually hitting them multiple times a week over time the cramping should fade, i know when i havent done abs in a bit or if i only do them like once a week they will cramp up sometimes. also dependent on some of the gear i might be using too.. shit like anavar used to make me cramp so bad i could barely do an ab routine which felt very counter productive to trying to make gains in that category.