I was found to have a dramatically low level of potassium immediately after my ileus in the aftermath of step 1, and later had a sodium overdose in the aftermath of step 2. The sodium/potassium imbalances are due to the removal of the colon and the period of readjustment. When I had the low potassium issue right after my ileus I was made to drink liquid potassium (it's deep red looking something like blood and it has an EXTREMELY VILE taste) to correct the situation. They stopped giving potassium intravenously because it is absorbed too rapidly and can cause cardiac arrest, and is absorbed more slowly (and more safely) by drinking it, even though it has a horrid taste. Eventually these problems all go away but for a while things are hairy.
I am not sure if your doctor explained to you but as mentioned by Jan electrolyte imbalances, which are to be expected especially after surgery, can cause these issues. Eventually the J Pouch gradually begins to assume some of the functions of the colon and your body learns to adjust electrolyte levels accordingly, but you recently had removed from your body the electrolyte regulator.
Hopefully you get tested, your diet adjusted accordingly and this will prove to be a temporary issue.