What Is The Reason For Sudden Change In The Behavior In Child?

Hi doctor, My son is 3 yrs old. Earlier he was very bright and talkative. use to be happy all the time. He use ti tell all the studies whatever has taught to him, alphabets, rhymes, numbers etc etc. He use to utter all difficult rhymes, numbers etc etc. his behaviour was very descent, very polite and obidient. Everybody use to tell abot his abilities and we use to feel proud of that. Recently from two-thee months his behaviour has become totally opposite. he dont listen anything,has become very stubborn, become very very naughty, not even studing anything, not even toched the books, not telling anything but still we know that he knows everything he is not telling anything, hiding from peoples, not mixing with peoples, not even telling hello to anyone. peoples are telling this is signs of autism. Please suggest some good doctors. Is it really that?????