it always feels weird to me when I play the notes on my violin.... unless the music i have is wrong, is it possible the song is either written in a different tonality, or is it just the singer adds to the song a little something special of his own?
With some very esoteric exceptions, songs are not generally written in a particular tuning system. It's very common for sheet music of popular songs to be simplified (or for performances of popular songs to be embellished, depending on which came first in a particular case).
Therefore, it's quite likely that "the singer adds to the song a little something special of his own." Most singers do. It's also quite likely that the sheet music you have does not agree with the recording you're familiar with, though that doesn't necessarily mean that it's wrong. Maybe it is: some sheet music available online is truly terrible. Or maybe it isn't, and it's just a question of some embellishments being added to the recording or removed from the sheet music.
A good ear training exercise would be to mimic the singer with your violin. A good notation exercise would be to write that melody down in precise detail and note how it differs from the sheet music you have.
Also, if you got the sheet music from somewhere dodgy on the internet, consider investing in a copy from the song's official publisher.