It seems like you are suffering from an acute exacerbation of ulcerative colitis. The inflammation of the colon is the sign of ulcerative colitis which is basically crohns that affects only the colon. It causes decreased potassium due to loss in stools and decreased absorption.
As you might know ulcerative colitis is a type of inflammatory bowel disease which causes inflammation of the bowel and causes severe debilitating symptoms.
1) Ulcerative colitis can be confirmed by doing a colonoscopy and a simultaneous biopsy. Histological exam can help in diagnosis of UC.
2) You might need parentral IV nutrition to improve the body stores of electrolytes and fluids
3) Take tab. Ondansetron or domperidone to decrease the vomiting and nausea.
4) Its imperative that you maintain a good, healthy and balanced diet. You will also need daily replenishment of important micro nutrients like calcium, folic acid and vitamin B12.
5) Get a colonoscopy every 3 years to rule out colon cancer.
If the symptoms persist for more than 2 weeks in spite of my recommendations, kindly consult a gastroenterologist who might schedule you for a colonoscopy.