guess some of you know the song "Grenade" from Bruno Mars, one of the lines is:
Should've known you was trouble from the first kiss
English isn't my mother tongue, but "was trouble" just sounds wrong in my ears. Is that some kind of slang?
In general, you can't expect pop music to contain the kind of language that an English teacher would consider correct.
There is a whole spectrum of language styles, ranging from dialect that outsiders can barely understand, through to the received English that is appropriate in most fiction, non-fiction, newspaper articles, essays and so on.
(That this has to be explained leads me to wonder how it can be any different in the cultures of other languages)
"You was" instead of "you were" is not "received" English. Yet talk to real people, and you will hear it all the time. And, indeed, the other way around.
You wouldn't be at all surprised to hear these in the pub. You would be astonished to hear a newsreader or continuity announcer say them.
I don't think it is sensible for a non-native speaker to attempt to learn dialect, unless they are already speaking received English to a very high standard.
No it isn't slang. Slang describes a particular kind of vocabulary, not grammar. Some varieties of English have a regular past tense for the verb be in that they use the same form, was, for all persons and numbers. Standard English is not one of them. In Standard English be has an irregular past tense, with was only for the first and third persons singular and were for all other persons. Foreign learners of English learn the standard variety, and that is why the line sounded strange to you.
“Should’ve known you was trouble from the first kiss” is grammatically correct African American dialect.