It could decrease for a number of reasons and/or medical problems. If you had fluid retention in your body the potassium would decrease which may not be abnormal, however, the underlying reason would need attention.
What would cause it to drop to 3.1, does that seem abnormal?
That level is on the low end, not dangerous low, however, it needs to be monitored. Important you do not go over 1500mg daily with potassium supplementation. Potassium should come from your diet on a daily basis and almost all foods have it. Fruits, vegetable, certain meats, and grains have the most amounts. The following foods have higher levels of potassium: figs, beans (kidney)-1 cup, avocado- ½ medium, molasses- 1Tbsp, orange juice-1 cup, banana-1 medium, milk (nonfat)-1 cup, salmon-3oz, sweet potato/potato- ½ cup, mushrooms- ½ , chicken breast- ½ , sunflower seeds- 1oz, and whole wheat bread- 1 slice. Potassium levels can fluctuate depending on the fluid you retain in your body.