Why is my dog getting vicious now?

ok so i have a baby jack russle he's 2 years old. and he used to be sucha sweet little dog, had him for some time now! now lately he's been getting so mean, he wont even let me go into my own room without chasing me, or trying to bite. and one time i was doing something and he just nipped my hand and it bled a bit. i dont know what to do, i dont want this to get worse why is he acting like this now?