You fought a war with her and after peace, gave her embassy? Yeah she doesn't like you and get ready for another war. Also it means you didn't renew a DoF that expired, if you had one
AIs are generally deceptive, I will talk more when I get a keyboard
First, when you said they denounced you, were you actual "Friends" or just on "Friendly" terms? This is very important, because one has a guarantee that they are being extremely ready to attack you if given the opportunity and the other is yes/no maybe.
Here's an example of Friends Denouncing/Going Neutral/Then Declaring War:
Generally speaking, the most dangerous attitudes for warring is with those supposed "Friendly" civs whom you just met, rather than "Neutral" civs who had denounced you beforehand because you can expect those to attack you within 10 turns of denouncement or so.
Because what you CAN do to immediately change the geostrategic situation to offset those "Neutral" warmongers is by either massively increasing your military (rushbuy) or do what I did with the Son of Gormsson there (a combination of both is just as good, btw):
And if they change that "Neutral" to "Guarded", you can expect them to not declare war on you. Guarded civs I have never seen them declare war, and if they want to, their stances will either change to "Neutral" or "Hostile"
There are a variety of reasons they may want to go to war that are not listed in modifiers - they are going for Domination or at least securing this part of the Continent, you are friends with someone they don't really like and this one doesn't show up in the list, they ACTUALLY covet your lands and wonders, you are a warmongering threat to the world etc.
But if you are ready to meet their attacks, then you should not have a problem in dealing with them! And remember - some civs can change their attitudes to you as the eras progress, so if Catherine backstabs and attack you in 1000 BCE, you might still have a chance of friendship in 1700AD or so!
edit: also what Smokeybear said! Don't be that odd fellow out in the Great Game.