1. Keep building, your core is cramping up because it's being subjected to shock it isn't conditioned to.
2. Drink more water.
3. Either eat more potassium or take daily supplements.
4. Add variety to your core workouts. Crunches only work certain parts of your abs, I do various versions of crunches, planks, russian twists, bicycle kicks(opposite knees to opposite elbows), flutter kicks and leg lifts. fukk situps though, they're a waste of energy and you basically would work your ab more doing crunches anyways because it keeps your body in that range of motion in which your abs would be getting the most work. I either keep my feet suspended a few inches off the ground with my hands behind my head and bust em out, or have my feet on the floor with both arms reaching up and out with my hands on top of each other, basically between my knees but a bit higher and do em, or do what I just mentioned but instead of your feet sitting normally on the floor, I put my knees apart kinda like a butterfly with my feet together, then lay back, and reach my arms out and do the crunches. You work more muscle groups this way, trust me....and you'll have to use more ab muscles just to hit the reps when you reach them arms out. Shoulder blades off the ground, tuck your chin, and press your tounge behind your top teeth because it helps to keep your neck stabilized and aligned seeing that these types of crunches also work your neck as well
Maybe don't cut out isolation movements but never prioritize them 
