Was immediately called "rusek" and "kacap" after disclosing the fact that I'm Russian while trying to communicate to Poles in their language.
Well, lets put some definitions first:
1. "rusek" is term wich means something between: "dude from east of the border" to "dude selling socks and old soviet uniforms on flea market". So it is really not so offensive
2. "kacap" is slightly worse, but it could be offensive or humorous, anyway this word has some interesting etymology:
Is there some kind of new-wave anti-Russian nationalists?
No, those are at least centuries old informal names for people east of the border in Poland, sometimes negative sometimes not. Those are quite capacious words. You can't expect that such old words disappear after just twenty years of normal relationship. I bet they will not vanish, but if things will be ok between us, negative association will vanish. That is the way usually language evolution works.
I do not joke about Ukrainians, Georgians, Poles, black people, whatnots.
Well, that explains a lot, because if nothing more happened then you are overreacting...
You don't joke about others nationalities? It is hard to believe that, but if so be aware that you are missing big chunk of human knowledege. Jokes about other nationalities are just another version of La Fontaine fairy tales... Of course in every country versions locals are the brightest and neighbours are not ;-)