Nope, not settlement limit, and not war. Too early in the age (this is around turn 10 or so I think).
But I I have no idea, I would be guessing that its the loss of policies and trade. I had a lot of trade going in the previous age, like 30/35 resources.
I really HATE the trade mechanic in modern age. Its great in the other two, but modern ages is horribly bad.
It just seems bizarre how you can come out of an age, doing very well, then its all a shambles in the first few turns of modern age.
10 turns later it was positive again, but it just seems so disjointed and when the UI is so bad at explaining what the hell is going on, its a little unmotivating.