You may not have intentionally broken a treaty, but did you unintentionally break one?
When the Mayans declared war on you, did that cut off a trade route between you and another civ you were trading resources with?
"..very bad terms." What?, do you mean they charge you too much? Luxuries are priced based on the # of citizens they will make happy. If you have a huge empire, then they will charge you a ton for luxuries because it would be making many people happy for you. If they have a small empire, then they won't pay you much for your luxuries, because it doesn't make very many people happy for them. Basically, you probably won't be able to trade 1 luxury for 1 luxury if you are bigger than them. You will have to give them 2 or more luxuries for 1 luxury, or give them 1 luxury and some gpt for 1 luxury. Oftentimes, if you are really big/have tons of population you would have to give them 6 luxuries (and some gold) just to get 1 luxury, which makes it not an ideal option (just take it from them if you are that strong).