A sourdough starter is a balance of bacteria which develops enzymes and Lactic Acid Bacteria (LAB). It is these, along with the wild yeast that it captures which produce gas. Switching flour introduces new bacteria and upsets the balance of the starter. It can take a few days for your starter to recover after the switch. If you want to continue baking with it you should separate a portion into a new jar and continue feeding both of them. Once the new one matures you can discard the old one.