RKU

my new 18month old royal python has changed overnight! the first few days i had him he was very docile and didnt mind handling and one night i saw him sittting with his head out of his hide staring at my every move! The next day I tried to get him out and as soon as I touched him he hissed at me, coiled his neck up and tried to bite me! Now every time i go in to get him he stikes out at me! What is going on? I would seriously appreciate some help.
Thanks