Help Me!!!! I'm desperate!

So I have an almost 2 year old (AJ) and a 3 weeker (JT) at home. AJ has been an excellent sleeper for over a year now. We put him to bed, he goes right to sleep and sleeps all night. Not a peep or a fuss. He takes good naps and sometimes even puts himself to bed.

For the last week or so he screams, and I mean screams, like he is terrified or in pain or something every time we even mention going to bed... sometimes he will go down just fine then start screaming in a couple hours.

I don't know what to do! Please help!