While people do occasionally have allergies to things that are very common, having an allergy to potassium itself would be incompatible with life. This is because potassium is a necessary part of so many of the processes that our bodies have to do every minute, that the human body could not function appropriately without it. There are, however, some foods that contain other molecules and chemicals that can be added to potassium to make them more allergenic. In these cases, it is not the actual potassium that is the problem, it is the sum of the other molecules all added together that makes a combined product that your body can not tolerate. The most common signs of food allergies are itching, rashes, runny noses, and other such annoyances. There are some people, however, that can have life threatening reactions to some things that they may eat, often due to swelling of the soft tissues in their upper airway which makes it impossible to breathe. These people often require doses of a medication that is called epinephrine, and they may even have to carry the medication with them (an Epi-Pen, for example). Please avoid foods that are causing problems for you, and speak to your doctor
about your questions and if you should be tested.