Calcium may be the problem nutrient, not potassium which is usually lost due to vomiting, diarrhea or diuretic use (water or high blood pressure pills). If none of these indicators fit you and you have normal functioning kidneys, your body will get rid of excess potassium from the supplement and bananas. Have you read my potassium topic?
Do you drink 2 to 3 glasses of milk per day or consume at least 800 mg of calcium by eating dairy products? If not, would suggest you take a calcium supplement and read the calcium topic. High intakes of phosphorus (meat and carbonated beverages) can increase the loss of calcium.
Also, would suggest working with an exercise physiologist and making sure you stretch out your calf and foot muscles before AND after swimming.