There are many reasons to it. It can range from stress, illness, fear, aggression to boredom or unadaptibility of environment.
Consider if any stressors or recent household changes could be affecting Tweety. Has anything changed in Tweety's usual routine, diet, environment, or handling lately? If so, you may have found the answer.
Even when you have changed your bird's normal daily routine food, that might also cause upset to your bird and it might let you know, like screaming, to change the food.
Sometimes, birds might even scream for companionship. It might be hinting you to get a partner for it.
A change of environment is something very common for bird to have side-effects. Some birds, when being changed to another hanging corner can cause them to lose appetite.
Numerous avian training and care books exist to help you with behavioral issues and good articles.
Do seek a veterinary examination to insure your little birdie is in good health.