Toothpaste companies place an ingredient in the paste known as sodium lauryl sulfate which helps make it all foamy when you brush. It also helps to clean your teeth. This ingredient temporarily causes your taste buds for sweet flavors to turn off. This way, all you taste are the sour and bitter flavors. If you drink orange juice, your taste buds only sense the bitter flavors and not the sweet flavors you are used to.
You can always wait an extra few minutes between brushing your teeth and drinking orange juice to prevent this from happening.